Tough cucumbers are usually the product of insufficient water, or nitrogen. They feel that they have limited resources to work with, so they ripen their fruit and seeds early, to reproduce. Also, there are a few naturally small kinds usually used for pickling, that become tough if left too long on the vine, hoping for a slicer. If those aren't the case, double check your seed source, and the descriptions of the variety you thought you had. Some seed companies will refund your money if the variety is wrong. If it is seed from a previous crop, it may have been pollinated with the wrong variety, and the genes of the parents aren't working well together.