Bitmap editing problem

My problem is that I crop a bitmap OK, then I go to edit it (eg erase a part of it). When I have finished it resorts back to the original uncropped image with the erased bit showing. Sometimes allows me to start to crop it and then when I click OK, it resorts back to original. I have used this same method for years to edit successfully, so can someone tell me what is happening and/or how I can resolves this? Should I delete casche or something?