Withering leaves on chillies

I have been growing 4 different species of chilli peppers in my greenhouse, in pots. Whilst I planted the seeds quite late (mid May, owing to the fact that I was building my greenhouse!), 3 of the plants have started to produce chillies.However, the leaves are withering a bit at the moment, despite keeping them watered, and there is no sign of whitefly/greenfly etc. The actual peppers look healthy enough.Any ideas what the problem is, if any, or is this usual?