This is most likely to be caused by inadequate pollination or irregular watering. You could try hand pollinating the female flowers with a male flower (pull off the petals of the male flower and push the anthers into the female flower to dust the pollen on the stigmas) or use a little soft brush.
If there are plenty of insects around anyway, then I'd suggest it could be irregular watering, as I've often observed this happening with squash plants which get dry and then get a thunderstorm. They would do better anyway if you try to adopt a drip irrigation system. This could be something as simple as using a large plastic water bottle and making a pinhole in the bottom and leaving it to seep into the growbag during the day. A steady supply of moisture to the roots will ensure the fruits grow steadily without being checked by drying out between waterings.

Thanks.