looks to me like watering and/or maybe a ph problem. do you have a ph meter? how often are you watering/how do you check if they need water?
usually if you overfert the leaf tips will turn brown, but it looks liek yours are turning yellow then drying out so I dont think its overferting. It is also normal for the bottom leaves to start yellowing and falling off as your plants pull the nitrogen out of em mid-late flower. However, it looks like you are more on the early side of flowering, so Im leaning towards ph being off, thus locking out nutrients. That or it could just be youre watering schedule needs tweaking.
My Aurora Indica looked very similar to your plants in terms of the problems, and my ph was way high and I also overwatered. Ive also heard that a high ph will keep your plant from drinking thus causing overwatering if in soil.
My advice would be flush em with some ph'd water.