Ours are doing very well. We've got cucs, courgettes, sweetcorn, tomatoes (several different varieties), peas and sweetpeas that have germinated. They are in a greenhouse which is heated on low temp nights.
If they are keeling over, you should be able to save them by transplanting them very deep. If they are keeling over and dying, is it damping down disease? Ie is it too damp and humid for them?
If your tent is too warm, that can promote leggy growth and also if the lights are too high above them or the light itself is dirty. If the area they are in is TOO perfect that can also be a problem. Eg, exposure to a light breeze is a good thing since that promotes thicker stems. You can just brush your hand lightly over seedlings regularly so that it stimulates the growth of thicker stems.
If none of the above and you are doing everything right, it is probably just one of those annoying things. Gardening can be extremely frustrating at times! I haven't had any "grrrrr" moments this year - yet!