Did you use new seeds each time?
At first glance, I was inclined to agree with Claire. However, the OP says he had the same problem last year. I suspect that it is, indeed, a pollination issue. Living on the fifth floor (I am assuming it is an apartment building) might be an issue, although I'm sure bees can fly that high. Do you live in an area where the city sprays for mosquitoes? That activity may serve to reduce the population of the insects that normally carry out pollination.
If that is the case, I am not sure what the solution would be. I reckon you could hand pollinate them but I have no idea how to go about that.
And try not to fly off the balcony....Looks like poor pollination to me, too, and that cuke is overripe. Those funny shaped cukes are fine to eat. If you still have flowers, it is not to late to hand pollinate--just make a little buzzy noise, hold the q-tip in your mouth, and fly from flower to flower. Don't sting anyone.