Hand pollinate in cold weather

If the weather is cold, the insects which would normally pollinate your marrow, squash, courgette or pumpkin may not do an efficient job.

In which case, hand pollinate the female flowers (you can spot them as they have a swelling behind the flower which will become the fruit). The male flowers have a narrower stem. Break off a male flower (the one on top here) on a dry day, and holding back the petals of the female flower tap pollen from the male into the female flower. Or push them together slightly.  A male can pollinate a few female flowers.