weak, limp plants

For the most part the girls are healthy but some of the stems near top of the plants are weak and limp. what could be the cause of this i will post pics later when the lights come on.

It could be they need water. Another thing is that the plants will droop there leaves on lights out. Let the lights hit them for about 20 minutes and if they still are droopy after about an hour you may need water. Pics would help.

The lights have been on for a few hours I feed them 3 times a day for 15 min in rockwool chunks. They are in veg right now.