Hey dude,
Couple of quick pointers, If in desperate need of some advice your best bet is always to look in the medical bay for similar issue's and if you can't find them, start a new thread in there, someone will jump in a help a lot sooner. Secondly, do your flush before trying anything else, if in doubt, flush it out.
If multiple leaves are yellowing all at the same time then there is more than likely a ph issue causing a lock out, nitrogen would be the first assumption, this would first be helped by flushing your plant, however, if it's just the ones on the lower branches, and to me that's what it looks like, then you've really got nothing to worry about. They can die off because of agge or purely that they aren't getting enough sunlight so the plant decides to disgard it and focus its efforts elsewhere. Looking at your plants I would say they are about the right age/height for them to just be yellowing naturally.
Also, when you say 'lightly sprayed them', what did you spray them with?? And and what point of their day did you spray them after lights on, mid-day, or before lights off??
The plant in the 5th picture, is that the one you've saids fallen over or is that LST'd??
:smokebuds: