possibly poor pollination. To get the best results don,t let anything grow for the first two feet so all the energy goes into the plant growth, if the fruit is not developing properly the take off every other cucumber till the plant gets well established.
Cucumbers need good drainage and plenty of feed and should be planted on a hill/mound so the stem is above the watering level to avoid stem rot. Like the tomato the roots need to breath so over watering causes them to clog and reduces the up take of water and feed which in turn reduces fruit growth