Rear rotor won't budge

[blue thunder]

I have replaced the front rotors with the drilled and slotted race concept rotors and cannot break loose the rear rotors. Well I just tried the left rear so far. I pulled the caliper free of the rotor and tapped the rotor with a hammer in the rear and also in the front between the studs. Its solid as a rock. This isn't the original. I had new rotors put on around three months ago. The kind that rust right away. Has anyone experienced this and do I just need a bigger mallet or what. Parking brake was not on. Any suggestions appreciated.

Mike