Posted: 25/09/2016 at 19:16
Are they in a soil based compost, or just multi purpose?
Also, have they been allowed to dry out totally at any time.? Once dried out in a pot, it can be hard to get them properly moistened again. Rainwater alone is unlikely to keep them well enough watered during summer conditions too.
If they're in a sunny sight, they may need a bit of shade. They're fine in some sun if the soil doesn't get too dry, but they get a bit anaemic otherwise.
It could be one of a few things, so if you can rule those out, it's a start