Hello Taproot!
Man you have to give a lot more information and pictures for accurate predictions what might be the reason, but i will try to help im not licensed plant doctor so keep that in mind.
You might be overwatering, this is cactus its not a tomato plant dont water every day or every other day, let them establish themselves, let them dry give the roots some oxygen you wont kill them with few days without water. Its better to be underwatered than overwatered cuz your roots can start rotting and you cant reverse that, if they dry too much you can just pour some water in and your good man, more water doesnt mean faster growth. And they are still young they are probabbly not even using all the water you are giving them.
The light source..T5's are good for start but you wont be providing them with enough light soon, switch to i guess HPS for example, get a exhaust fan and if you have an air circulation fan DONT put
it to blow directly on them. T5's actually should be kept really close because they dont throw light very far away the further away your T5's are from your cacti the less light they will recive which may stress them and you dont want that. The T5's wont burn the cacti unless its touching it, simple test you can make is put your hand under that T5 and ask yourself is it too hot for me? if its good for you its good for the cacti.
Soil extremely important. What is it? this may be key to your problem what it contains?? Share that
"I check the roots and everything seemed fine." i dont even want to imagine how you did this, dont stress them it will cause them stunned growth. What size are the pots? dont let them get rootbound this is another cause of stress and stunned growth. Soil pH? N-P-K ratio? are you giving liquid organic/synthetic fertilizers if yes what they contain? Is the soil Peatmoss based or sand or whatever? you got perlite in it? what is the temperature in there day and night? humidity check that.
P.S. - Believe me the only person that can help you is yourself, i say that because only you know all the things you did to the cacti's there are TOO MAANY things that could be the cause. Read and educate yourself im sure you will find the reason. Give more info and more pics. Good Luck :thumb_up: