they may not be getting enough nutes. What's your ppm's?
A nutrient deficiency looks like a cell collapse (the natural appearance of the firm leaves lose some or all of their stiffness), usually along with some form of discoloration and or wilting.The affected part may wither and die, but it should not look like nute burn. That is the major difference between a nute burn and a nutrient problem. In time, you should be able to easily tell the difference yourself. If you suspect it is a nute def', then check your pH. If your pH is not right, then solve the pH problem before you attempt anything else.