Are the plants in a hydro set-up? In a soil mix? If a soil mix just what is in it. What, if anything are you using to fertilize the plants? If you are fertilizing then when was the last time?
Right off the top the problem could be to much fertilizing. Or, it could be the plant reacting to an insect infestation on the newer leaves. But, without the rest of the info no one will be able to say for sure.
I do notice a couple of areas in the photos where the ends of the leaves were cut off. If that is true, why were they cut off?