I have four squash plants in two very large containers and although there are plenty of flowers and small small fruit still developing, only one or two are actually swelling. The others started to but then stopped growing completely. The plants have been well watered and fed at regular intervals. Any suggestions? Many thanks.
The ones that aren't swelling probably weren't pollinated or pollinated properly. They usually shrivel up and fall off.
Try pollinating with a small brush, i do this successfully with chillies ie gently brush inside of each flower with a small brush
For squash, you would need to use the brush to transfer pollen from the male flower (growing on a stalk) to the female flower (with the miniature fruit at its base). Best done early in the morning when the female flower is open and ready.