I find they do this when dry and not fed every week.

It could be over-watering, too. They do need a feed, depending on the size of them, but not much water. I take it you have them indoors? Have you taken them out of their pots to look at the roots? They may be pot bound, or perhaps something is eating the roots, i.e. the dreaded vine weevil, which love to get into planters. Annie (Cumbria)

I assumed they were outside plants and replied accordingly - please ignore if they are house plants as I dont have them indoors.