Only able to upload periodically

I am using WP 3.5 and occasionally have an issue in uploading photos. Some seem to upload fine (about 1 mb each) and then it stops uploading any more. The picture icon just sits with the “thinking” going round and round where the picture should be. It seems that one day a photo will upload and then the next day the same photo won’t upload. I am saving them through PS CS6 and saving as “Standard”. Last night I uploaded some and then it just began “thinking”. Then today I uploaded a couple more and it went back to “Thinking” mode for each attempt.
What would cause this intermittent functionality? I need to upload more than a couple a day. In the past it seemed to work fine.