Problem/Bug Prison Ranks + Donator Ranks (PEX)

Problem With The Following Plugins:
PermissionsEx
Rankup

Hello,

I am currently in the making of a Prison server and it is going really well; Although, I just have one issue with it. I am using BuyCraft for the donation ranks. In BuyCraft I made a package and put the command as '/pex user {name} group add Premium'; It would add the user to Premium. After it adds the user to Premium, it changes their prison rank all the way back to the start and will not allow them to rank up. For example: Notch was the R prison rank and he donated for the Premium donation rank. Before he donated, his chat looked like this: [R] Notch: Hello. After Notch donated for the Premium donation rank, his chat looked like this: [A] Notch [Premium]: Hello. After that, Notch was unable to use the /rankup command as it would say 'You are not allowed to rank up". I really need help with this A.S.A.P as this is the only thing stopping me to releasing my Prison server. Many people are asking "When is it going to be up? It's been forever!" and I just don't know what to say anymore. Please help me.

Thank you,
PrinceMC.