I've often had trouble finding a squash hidden under all the leaves until it was way too big, so this year I put a tomato cage over one squash plant just to see what would happen.
This page contains the following solutions.
The squash vine borer is a very insidious moth that feeds on squash plants. You can tell it attacked your squash when you can see that the stem of the plant itself has been destroyed and gutted out by a disgusting fat white grub.
If your squash is producing smaller fruit it can be the result of a number of things, such as watering, fertilizers used, the variety planted, and more. Trial and error may help determine the cause. This is a page about what to do when summer squash is not growing large.