Incomplete pollination. Due to lack of insects and/or extreme temperatures, be that high or low.

You can be the bee -- move the pollen daily, in early morning before the flowers close.

Jean: forgive me my ignorance.With incomplete pollination can the zucchini start growing normally and then go rotten at one end? I thought that it would not even start to develop if not pollinated???
My morning routine is very busy and could not do that.. The zucchini flowers are open all day in my garden? I can see them all day opened? Thanks..and I btw I have enough zucchinis so its ok just curious..Thanks again.

Could it be blossom end rot? This can be caused by a number of different things, but in my experience it's mostly uneven watering.

You asked:
"With incomplete pollination can the zucchini start growing normally and then go rotten at one end?"

Yes.

You wrote:
"The zucchini flowers are open all day in my garden? "

Not the males.

Thanks Jean: You stand correct about the flowers they are not open day time.I must have been looking in the morning..
Thanks Barbe to you also.Uneven watering that it is because the leaves create umbrella like situation so my sprinklers can't exactly water everywhere.It would help if I did it, but I have enough zucchinis so no big problem.
Thanks Jean for correcting me..