B Drum. Its so hard for me ? Can someone give me some advice ?

For example, i can play Foo Fighters - Rope and Nirvana's - In Bloom perfectly...except for the b drum. If i play the b drum along with the drumming, i mess everything up. It completley throws me off and im getting sooooooooo frusterated right now. Does it ussually take long to get it down pat. Ive been playing for a month n a half roughly. Ive picked a lot of stuff up really quickly in my opinion. I mean people have seen me play and say that im nailing the drums but again, with the b drum. What is wrong here...?