Maybe you didn't use enough vitamin C or leave it on long enough. I had several rust stains from nails that were thrown in the pool during construction of our house. I put a whole bottle of vitamin C in a sock, tied it to the bottom of my long pole and left it sit on the stain for a while. Then I actually scrubbed at the stain with the sock. It took a while, but first the color of the stain changed to more blackish than rust and then went away. I can't believe the help I've received from this forum! I also converted this summer to BBB from Bacquacil and my pool has never looked better! I can't thank enough all the people here who post and help!