how come I have small zucchini on my vines for weeks, but they do not grow any longer than about 2-3 inches?

Answer 1

They either didn't get pollinated or they got too hot & aborted.
Poor pollination affects fruit set. Too much nitrogen can also delay the setting of fruit on the vine. A few other factors can affect fruit set. The most common is a mid summer heat wave. High day and night time temperatures will cause plant stress. The tiny pollinated fruit may abort as a result. A heat wave can also deter bees from their job, making hand pollination more important.

You can pollinate by hand .Take the male flower and gently rub its pollen onto the stigma sections in the center of the female flower. Pollination will be more successful if several male flowers are used to pollinate one female flower.

If the weather has affected fruit set... cooler weather will correct that.

Usually when they abort, they shrivel up & drop off.
If they are alive, which it sounds like from your description, then they are just slow growers, & should speed up growth soon.

Good luck! Hope this helps.