Cucumbers bloom then turns yellow and rots
I have what looks likes a healthy plant. It produces blooms and when the cucumber is producing, it starts turning yellow and falls of the vine. This has happened the past 2 years. I change the soil every year and in the past they have done great. I mix potting soil, Garden soil and this year I added a touch of Black Cow. Don't know what to try next.
It sounds very much like you are having a pollination issue.
Hand pollinating will help you correct this issue.
Here are some articles that will help you.