I'm not going to try to diagnose it, that can be a bunch of different things. But if you post pictures of your plants and also the temp and humidity of your grow space, and even what kind of lights and how far away, someone more experienced can probably tell you exactly what's going on. A lot of different problems have very similar symptoms. For me, when I had something similar, the cause ended up being bud rot. The humidity in my tent was getting into the 60% rh and i had some fat colas that didn't like that