A few suggestions: Firstly you may have planted snow peas, which have flat pods and very tiny seedlike peas inside.
If you do have ordinary garden peas, then try giving them plenty of water which may help the peas grow and swell up — that's if they have been lacking in water of course. Sometimes pea pods seem to be large enough, but empty, and then finally the peas fill out!
I know climate can affect pollination of most plants, including peas. Peas self pollinate, but sometimes all or some of the seeds inside don't become fertilised because of a sudden cold snap particularly if it follows a hot spell.