Could it be the lack of a good grow medium? Could rockwool stunt the growth maybe because the roots can't pentrate in (because my rockwool is damaged). Is that possible?
There's nothing at all wrong with rockwool, it's some good stuff no doubt. I highly recommend it as a growth medium. The rockwool you have is damamged and the roots can't penetrate it, very similar to growing in hard clay soil.
Rocks and coir would both be good to grow in but they require completely different management techniques. You still need something to germinate a seed in as neither coir or rocks would serve that purpose very well. If it were mine I'd toss the starts you have, take a knife and cut one of the blocks into chunks, say 2" x 2", and start over being careful not to smash the rockwool. Keep it moist during germination.
Nutrients aren't necessary for germination and can actually do more harm than good until you see the first set of true leaves forming. I've got 150 or so lettuce seedlings going now and they haven't received anything but tap water. I'll add the first nutrients tomorrow (day 6). Can't say if it's half strength or what as I use a ppm tester and mix a ratio of 1 part Boost 2 parts Grow until I reach the desired ppm level.