Hibiscus leaves problem
 

I have a few hibiscus in pots indoors in a south facing window. I have
quite a lot of leaves turning yellow and then dropping off. Could the
cause be not enough plant food?

The year before last I over-fed them with phostogen powder plant food and
some leaves went all wrinkly and scrunched up, so perhaps I am now being too
cautious food wise?

If you have an idea of what's causing this problem I would be grateful,
thanks.