I have lots of flowers on my plants and very small cucumbers, about 20 cm long, but thay never get any bigger. I keep them watered well in the greenhouse and feed with tomato food.
Certified GKH Gardening Expert
It sounds like a pollination issue. You will need to hand pollinate. Cucumbers are identical to melons on how they pollinate, so this article will also apply to cucumbers: https://www.gardeningknowhow.com/edible/fruits/melons/hand-pollinating-melons.htm