Q.Zucchini
I have been getting lots of flowers and zucchini to start, but when they get about finger size, the tips on many of them have been turning yellow and then they seem to rot.

Here are a couple of articles that discuss possible causes for this kind of a problem: https://www.gardeningknowhow.com/edible/vegetables/squash/squash-fruit-falling-off-the-plant.htm
https://www.gardeningknowhow.com/edible/vegetables/squash/squash-blossom-end-rot-causes-and-treatment.htm