This is a close up of the same slow tree.

Are there any differences in conditions at the site of each crApe myrtle? I know you said they were only 10 feet away, but drainage, sun light, etc could still be different at the two sites. I notice that, in the first picture, the grass seems very spotty. This could be the result of weeds in a winter-dormant grass, but it makes me wonder if it's a sign of problems. Are both crApes the same cultivar? I can also see that the first crApe myrtle has been absolutely butchered. What's up with that?

The one had been hit by a car, branches were broke off. Much of the lawns are splotchy like that here in FL. If it is a problem with the grass its very common. In the second photo you can see moss growing on it, so I wonder if that's an indicator of bad drainage.

I see a lot of lichen. The green stuff may be moss, I can't tell for sure. The overall picture does appear to indicate a very weak/low-vigor plant.

Here's what I would do:
First, cut all trunks down to about an inch or two above ground level. This may seem drastic, but is not a problem for a healthy crape myrtle. Yours will either respond well to this treatment (grow new healthy trunks and possibly experience in increase in vigor) or not (a really weak crape might not survive this, but probably wouldn't survive very much longer anyway). Now is the ideal time for this treatment. Your crape is in decline now, so, if it's going to live much longer, something needs to be done.

Next, over a period of the next week or two (or longer if your weather conditions don't change much over that time) check the moisture level at each crape. Stick a trowel down into the ground and check at a level of a couple inches below the surface. My thinking is that, since you are in Florida, the drainage may be fine, but that's not necessarily the case. Is your soil sandy or clay or something in between? You may already have a good feeling of what your soil's like, but we don't without info from you.

If the site of your weaker crape is bad (poor drainage or whatever else), you may want to consider moving it. Crapes are usually pretty easy to move (although the remaining root pieces commonly sprout up for a while), but there is a possibility that yours wouldn't survive the transplant. If it doesn't, you shouldn't feel bad, because it was on its way out anyway and not moving it wouldn't have been any better of a solution.

We have very sandy soil here in FL. I had wondered about cutting it. I have seen them sprout back up after cutting. I don't know that much about trees tho so thought I would ask. Because it was planted as a memorial tree we would kind of like to get this tree to be better.

I have 3 crepes about 10 feet apart in the back and the largest one is significantly larger than the smallest one. My largest has lichen and it is doing just fine and has not needed to be treated. The one crepe may be blocking some sun from the other. my smallest one is slower growing due to less sun.

I would check the roots out, i cant tell from the pic but it does look like that smaller crape may be planted too deep, soil should be even with the root ball. None of my crapes have room for grass to grow in between the different trunks. once you expose the root you could take a picture to post where the soil line is on that tree compared with the root system.

Crapes don't care if they are planted shallow, my most thiriving is the most shallow. But a 4ft crape can be transplanted easily it if it's too deep.

also you mentioned it getting hit by a car. when a crape takes damage it will stunt growth for a short period. my middle sized one got struck my lightening and split about 1/3 of it off, but that 1/3 i left attached to the tree hanging by little hoping it would die off and it kept it alive for over a year and bloomed, but looked ridiculous so i eventually cut it off and it did not grow at all that year.

If this were my memorial tree, I would replace it with a new, vigorous tree that wouldn't hurt my feelings every time I looked at it. Be sure to purchase a named specimen...not something labeled 'Red Crape Myrtle', and give it plenty of room to grow. At ten foot centers, your trees will be growing into each other as they mature, you know. Perhaps that's what you want.

That particular tree isn't the key to the living memorial. It's just a tree....and a pretty sad one. Plant something beautiful and healthy to take its place.

.... my mothers tree seems to be stalled. It gets flowers and leaves. After 18 years it is only about 4 ft tall.

Okay, I'll ask the obvious..... are you sure it is not a compact or dwarf variety? Aside from being beat up and moss laden, sounds like it is leafing out and flowering as it should. There are MANY varieties cultivated to top out in the 3 to 6 foot range... and of course, it may be compact even if you thought you purchased a standard size tree. Even with the best nurseries, stock occasionally gets mislabeled somewhere in the supply chain.

The following TAMU site is one of several on the Web covering the numerous sizes.

Here is a link that might be useful: select crepe myrtle varieties grouped by size

Consider the fact that the one not thriving appears to be planted too deep in the photo you posted.

Where I live, they don't like that and rather than grow like a weed, they just sit and struggle.

'They' will tell you to plant it at the same depth or higher than pot, but if it's wrong in the pot.....

Here is a link that might be useful: PDF from USDA

Thank you for taking the time for all of the info and suggestions! There are lots of things to consider here.