Without more details from you, it could be many things. I also have the LHi card and CS 5.5 and have it working pretty well.
Did you use an AJA sequence preset? Are you getting video monitoring in the preview window and not the timeline? Have you checked your cables, connections, etc? Are you using the AJA player and not the Adobe Player in the playback settings? (Look in the preferences, and also you can right click and set the playback settings in the program monitor.)
I have found AJA tech support to be great- try them as well.
Good luck.