you need to remove the nub from the bucking, you also need to remove the lock ridge from the bucking,
Most flat hop nubs although claiming to be drop in do need to be cut down length wise, a particular way.
As much as I hate to do this amped airsoft does have a half decent flat hop guide, its not the end all to be all guide, but it will show you how you should be cutting the nub.
https://www.youtube.com/watch?v=ZqXbVQshNpI