It's common for squashes, zucchini (courgettes) and pumpkins to produce only male flowers at first (the female flowers are the ones with a swelling behind the bloom). If this is what's happening, be patient - they'll start producing female flowers when they're ready.

If you have both male and female flowers on your squashes but no fruits - or if the fruits begin growing, but soon rot and drop off - it's likely to be an issue with poor pollination. Make sure not to use any pesticides, which will kill pollinators and other beneficial bugs as well as pests. Pollinators will also be scarce at low temperatures and in poor weather conditions.

However you can hand-pollinate your squash to help things along. This video demonstrates how to do it: https://www.youtube.com/watch?v=4Baj4vUXFMA