This has happened to me before. What happens is that on certain grips, the hole where the spring fits is drilled too shallow. Your spring is then compressing more than it should and therefore causes your selector to be extremely stiff. You actually are able to move it, it just may take 10-15 lbs. of force and may even damage the detent or selector. Filing down the detent is the best idea posted so far and this will help if, and only if, you're off by only a bit.
The best solution though is to cut off a link from the spring. This will reduce the tension on the spring when it is compressed by the detent and safety. Usually one or two links and a wire cutter will do the trick. So far I've gone through 3 different grips on my AR and 3 different lengths that I've needed my spring to be. Go figure.
One last thing, before you cut, make absolutely sure it's not an assembly problem. It's much easier to check twice and cut once than have to find a store that sells AR parts in California.