[Haskell] class instance lost during re-export & packaging

I have a package P that contains a class instance specified in one module P.M and re-exported in packaging module P (one that gathers & re-exports for the whole P package). When I import the package module P in a test module, Under ghc-6.6, my instance is not found. If the test module also says "import P.M()", the instance is found. Oddly, I only have this problem with classes defined in a package other than P. In ghci, after loading the module successfully, if I comment out the "import P.M()" and re-load, the instance is found. Is this behavior known, and perhaps even intended? Is there anything I can do to make sure my instance gets packaged up with module P? Thanks, - Conal