Fins not healing up?

Hi all, firstoff I'll say this is my first post. I've done some reading and it's been informative! I've been having an issue with my Silver Dollars though for awhile. Normally when my fish get nipped fins or what not they heal up in days. My Silver Dollars were my first tropical fish (except for my first betta) and within my first two months of having them they got their fins nipped by a Blood Parrot I used to have. I didn't think much of it until I noticed they never healed. I didn't see any signs of fungus, but I thought maybe it was bacteria. I put them in a hospital tank for about a week with aquarium salt and some Melafix to no prevail. After a few months I just gave up. Now I've had them for almost a year. I started thinking about it again because last weekend I stopped by a fish store that had an adult Silver Dollar and it looked so much better with normal fins and being a little bigger. It got me thinking about it again and I'd really like to know if I'm the only person with this issue. Like I said in the past I've dealt with fin rot due to fungus to just torn fins from regular nipping and it's never been much of an issue. It's been bugging me thinking about it again!