I had problems with Leupold quick release base and rings with a Vari-X II scope on my original T/CEncore.Tried everything, kept shooting like 2 feet low. Sent the scope back to Leupold to have them check it, they said it was fine. Tried it on the Encore again, so problem. Put the scope on another gun, it worked fine! I gave up on having a scope on my muzzleloader that season. Mounted a Nikon Monarch on it the following year, no problems. About 2 years later I read on Modern Muzzleloader that for some reason on certain muzzleloaders Leupold scopes need to be SHIMMED to get the height they need!!!!!
I don't know what else to tell you. You can either switch rings or mounts or try another scope. A guy at work just told me about the same problem a friend of his is having. (Not sure of the gun type).
Believe me, I was EXTREMELY FRUSTRATED with this whole ordeal. I know what you are going through. Leupold makes great products, I prefer their mounts over others. I have made the switch to Nikon scopes, this situation was one reason why.
You could try calling Leupold, They may send you the necessary shim.
Good Luck. Hope you get it sighted in.
Mike