I doubt that it's a fertilizer issue. If the plants are otherwise healthy & vigorous, chances are all of the nutrients are there.
Without double checking-- someone correct me if I'm wrong-- here's what you might be seeing. I *think* cucumbers are like squash in that there are male & female
flowers. Pollen from the male needs to be delivered to the female
flower. But typically, male
flowers blossom earlier, before female
flowers. So you may just be noticing this--
flowers that bloom & then fade away-- because you don't have a lot of female
flowers yet. As soon as the female
flowers bloom, you won't notice it, because they'll be pollinated & you'll have cukes. And since you do have some cukes... some females are showing up & are being pollinated.
Could you post a photo? Believe it or not, there is a way to tell the difference between boys & girls! The flip side is that there could be something wrong!!