I was coming here to post the very same question. 3 plants out of my 18 just aren't moving much. One is a variety i gew last year, and it was extremely vigorous. Any thoughts on this "failure to thrive" and when to yank and replant?

You can only compare them to the other plants of the same variety when evaluating performance. Other wise it is an apples to oranges comparison.

So are they in ground or in containers? What, how much, and how often have they been fed? If in ground, what soil prep was done? If in containers what soil-less mix was used? Is there a low lying spot that could be collecting water near them? Sun exposure the same? Any chance of over-watering? Chance of root damage from stake or cage? etc. etc. Environmental issues in other words.

We always make the mistake of laying the blame on the plant when 9 times out of 10 it is something we are or are not doing that we need to fix. :)

I have 6 Black Krims in and while they are somewhat smaller than many of my other varieties and IME will remain so they are performing equally.

Dave

Mine are in the ground, 3 of the other Krims are 18 inches or so, none have been fed fertilizer, but the soil was amended with very well aged horse manure over the winter, all were staked/caged when they were put in the ground (cages to save them from the chickens, doing a Florida weave for support.) One of our gutters has a leak but we have a tub under it, so no over watering. Same sun as the rest.
Since they show no sign of sickness, I am just going to let them do their thing.
This is the one space that is not a raised bed with hardware cloth bottom, could a mole/vole be a problem? Cats have been bringing me trophies for a week or so.

I am having a similar "issue" allthough it's not much of an issue because I planted about 100 more seedlings than I can use. I am giving the rest away to friends and our community gardens. BUT...

All were started in the same flat, 2 of 7 rows did terrible in the flats, while 5 of 7 looked exactly how I wanted them to look. I potted up into the solo cups with pro-mix, scotts potting mix and Miracle grow potting mix all mixed together in a large tote.

They were in my south and west facing sunroom getting good light with a rotation under my 4 T8 bulb setup. Each day I would put about 30 under the lights and rotate them every day. It was the best I could do for this year.

I started putting them out about 2 weeks ago and the wether has been favorable, other than more rain than I would like. But, they spruce up quickly after each rain and have even made it through a couple good dounpours. They have also made it through 2 days in a row of 75-80 temps and not a cloud in the sky full sun.

My biggest few plants have nice thick stems, 3 sets of true leaves and are about 4-5 inches taller than the top of the solo cup which was filled about 3/4 of the way. Then the majority are about 2-3 inches over the top with thier second full set of true leaves.

Then about 10 are 0-1 inch above the top of the cup with only one set of true leaves. The suprising thing for me is that my daughter started some of my marglobe seed about a month ago, a full month after I started my flats, she bought one of those peat pellet starter domes. All of her plants are as tall and have better looking canopy than all but the best of my other plants. They never went under the shop lights though, only the first 2 weeks in the sunroom (including the first 5 days of germination) and then also spent the past 2 weeks in the elements.

I am not too worried about any of them though, save maybe the 10 that are really small. I just recently planted out 18 Romanesco broccoli plants which also had varying degrees of size and healthy look about them. They have been in the ground about 3 days and they all look almost identical now, they have all double to tripled in size since I put them in. The smaller ones tripling to catch up to the larger ones which may have doubled.

Now if I hade them in the ground and they had not moved in some time, I would in fact be a little worried. 3 weeks is certainly too long for a healthy tomato plant to not grow at all. I would personally dig one up carefully, find the root mass and massage it loose, then I would re-plant just to be sure it isn't rootbound. Also what is your soil like? Are you planting into clay? I don't have clay but have read where if you dig a planting hole and amend just the hole, the clay can be so much more dense that the roots could just swirl awound inside the planting hole taking the path of least resistance, essentially giving you a potted plant.

You can have, what I call, a bad spot/s in the soil for reason/s you will never know or are worth fussing with.

I have had plants do that and I either use a shovel to dig out the plant with a "root ball".
Then I put new bagged soil apprx. a foot deep and a foot around and put the tomato back in the ground with a foliar feed that has root boost in it.

I have only had one plant in the past ten years that was hell bent on dying.