What are your watering habits? How much light does it get? What type of medium is it growing in? How long have you had it?

When you up potted, are you positive that there was no root damage? Especially to the fine root tips.

I water when the soil is dry, about 3" deep, probably once a week. It is by a window, and has grow lights on for about 10 hours a day. I have it citrus potting mix mixed with orchid bark. I have had it for about 18 months. I did not see any root damage at all. Thoughts?

What kind of pot do you have it in? Do you check deeper than 3 inches? In a large pot the top few inches can be dry and the bottom can be wet still. What direction does the window face? What kind of citrus potting mix?

I have it in a deep ceramic pot, the moisture probe goes in about 10". What should the probe read when the tree needs water? The window faces east, but my only other choice is a north facing window, I do have grow lights. I used Miracle Gro citrus potting mix. The tree was doing very well for the first year, it had new growth, flowered, and produced a few lemons, although they never ripened. Then, the leaves just began to drop. Now it's close to being just branches, and I would love to save it.

I can't speak to your moisture meter, but whenever I use a moisture meter to check my pots (I like the convenience of being able to easily get it clean and just see the moisture level at a glance), I first calibrate to figure out what it reads as "wet" and "dry" by using a sharpened dowel to check the moisture level. Whatever the meter reads as wet when the dowel is wet and whatever the meter reads as dry when the dowel comes up dry is my measure of wet and dry. Then I occasionally verify my meter with the dowel.

I would be careful of the miracle gro soils, between added moisture crystals in some and the fact that most of them just retain too much water, in addition to other factors, I just haven't had much success with them. If you want to use bagged, and If you can get it, keloggs has a decent soil, I've found G&B organics citrus soil a good option, and black gold makes a decent one. Others here might have other suggestions of other brands that are easy to track down.

What's the temperature in the room where you have it? Does it see a drop in temperature at night? What are your humidity levels like?

MG soils are all garbage! They are all peat moss, including their Palm and Citrus or Cacti mixes. They become extremely hydrophobic in real sunlight and heat (almost impossible to get them wet), and indoors, it's nearly impossible for them to dry out!!

Thank you! The temperature stays at a constant 70 degrees. The three potting soils mentioned are not available in my area. If I were to mix my own soil, what would I add, and in what proportions?

Here's a good link on soils for citrus trees. There's also a decent discussion by Al on here of soils, but I don't have that link. https://www.fourwindsgrowers.com/blogs/four-winds-growing/soil-mix. Four winds also has some other good information for growing citrus indoors. I prefer mixes that have pumice in place of pearlite and recycled forest products in place of the peat as they seem to do better for trees and other plants, but it's really personal preference. If you can, I would allow for a temperature drop at night for the trees, citrus tend to like a temperature drop of about 10 degrees at night. Indoors I would also get an oversized drip tray, fill it with lava rock or other large rock (this is decorative, so it really depends on your style, what you want is rocks large enough to allow all the water from watering to drain into the bottom tray without touching the bottom of your pot, so that the water doesn't reabsorb into the soil. You could also use pot feet inside the tray and it would do the same thing). What this does is it allows for an increase in humidity around the tree as the water evaporates without needing to use a humidifier. It can really help to have. And when I say oversize, I mean for a 14 inch pot you want at least a 20 inch tray. For my big citrus I use steel trays on rubber castors, for my smaller ones I just use the standard vinyl drip trays. A bigger tray also works well, if your tree does well, catching the occasional leaf and spent flowers so they don't wind up on your floor.

Thank you so much! I will definitely follow your potting advice, and try to drop the temperature at night. Fingers crossed!