I have the same problem on a PowerBook G4. I took it in to get looked at, since it appeared that the drive hardware broke somehow.
What was happening to me was this: First, the drive would spit out any disc I inserted. Then, it wouldn't even accept the disc, and "feels forced" when I try to put it in, exactly like you describe.
One thing that can cause this is a foreign object or miniature disc stuck in the drive - slot-loading SuperDrives don't like the miniature discs too much.