One thought which may apply: if you had a lot of rain then it may have stopped the local bees from working and you had only partial fertilisation of the flowers.

If you had a fine spell later then flowers from that time may give better fruit - provided the bees were out.

For next year - remember to feed very well with good compost in autumn and prune back to encourage the new long shoots. Treat the bush just as you would for a rose that you want super blooms from (same family).

verivert8,

Flowers are either fertilized or not fertilized. Just like humans, a woman is either pregnant or not, she can not be "a little pregnant."

tamndart,

I think the problem is the result of a lack of soil moisture during fruit development. If most of the rain occurred in the spring and then in mid summer, the spring drained through the soil and the mid summer rain was to late.