Oil the catch at the sides of the slot/roll pin, and at the bottom of the lever/spring, then work the catch by hand. It should loosen up. It also helps to use a empty mag in the mag well to add in the process. Once worked in, the empty mag itself should push up on the catch.
If you find that even after the catch has been lossened, the catch will still not rise with any of your empty mags, then you may a problem with the catch spring, spring detent pin, or spring channel. If this is the case, use a punch and punch the roll pin out of the catch. The 3 pieces will be a catch lever, a small spring and a detent pin. Check the spring channel/hole for crap that may be in the hole and causing the catch return spring to be too strong for the mag to overcome. Most of the time, it is just a flake from the anodizing process that is causing the spring to be over pressurized, but depending on the receiver, the spring/detent may be too long or the hole drilled to shallow. Also, any small flakes in the hole will be crushed when you break in the catch by hand, so don't worry about having to pull the catch to clean them out.
Note:The spring can be trimmed if needed, but this is a worst case scenario.
Hope this helps.