Variety? Some have very small pods. If they are not filling you may have a phosphate deficiency, but could also be a Ph imbalance.

Do I understand that this is a bed that has produced peas properly for three years, and just this year it is not doing so? If so, the problem is probably not nutrients. But if this is your first time growing peas, phosphorus deficiency is indeed a possibility, and you might get your soil tested to see where you stand on everything else. Phosphorus deficiency will impact many fruiting vegetables.

Peas are mainly self-pollinating, and it's also possible that you just had some unfavorable weather when the peas were flowering that interfered with pollination.

hillseeker - where are you located? Zone 3B would have me guessing somewhere in the interior of Canada or northern Europe. It could be lack of sufficient sunlight this time of year causing your issue. I'm assuming you're growing them in some type of heated enclosure here?

Or maybe you aren't actually growing them at the moment and are just inquiring what might have caused the issue last season?