Could be sunburn, but yours don't seem to be getting too much sun. Make sure they aren't getting a lot of reflected heat and are not behind glass. Could also be overwatering, they don't like to sit in soaking soil. Could also be fertiliser burn, they like some fertiliser but not very strong fertiliser.

Are the edges of the leaves brown and crispy? Do the leaves show yellowing and drop while still quite firm? Do the leaves show discolouration from the edges inwards, or patches all over the leaves?

I am having the same problem and am on my second round. The leaves start to show discolouration from the edges inwards then turn crispy and brown. Mine are in sun for only about 1 hour day and are getting adequate water. I am thinking that maybe there was something in the potting soil I used that they do not like and maybe thay are getting burned. My leaves stay on, get wilty(even when they have just been watered) then the bottom leaves start getting brown and crispy. Any suggestions?
Thanks!

My zone is Cent N.Y. My plant is healthy, but no longer puts out flowers. Can I do anything?