So I've done a few uploads of my images and well out of 4 uploads 1 was selected. Fotolia accepted my stapler image but Dreamstime complained about exposure and contrast. At least they give more details as to what is wrong with your image when compared to Fotolia.
Many stock photographers have said never give up. Most of them only got accepted on the 4th try and even after that still received many rejections (its heart breaking i know) but after a few months of work and dedication the amount of images that they have had accepted has grown.
stock photography is not that easy! you have to think in terms of the reviewers and the buyers. If you don't you'll just be wasting your time. So be patient and work hard!