2 things you can do. First go buy some 33rd wolff replacement magazine springs. You can jam them into a 17 round mag body. It cuts capacity down to about 15, but the extra pressure on the follower will usually overcome the resistance of your grip on the slide catch. If that does not work by itself you need to build up the shelf on the follower with JB weld. Use masking tape on a clean follower and after the JB weld has dried fit it so its not causing premature lock back. Then pop the JB weld off (it doesn't adhere to plastic very well) put on a dab of epoxy and drop your little JB weld plug back onto the follower shelf. That in conjunction with the 33rd spring will guarantee slide lock.