I have had similar problems in the past with both hydroponic and soil grown tomatos, and I have found that if the bees are not about, I assist mother nature by getting a cotton bud and giving a gentle wipe to each flower bud then it will start to fruit.
Another area could be the fertiliser, but I have grown tomatos under some really stupid conditions, but the cotton bud seems to do the trick. I had the same problems with Egg Plant (Aubergines) and had the same results. Good luck and keep us posted.