02 Avalanche Bed Cover

I own an 02 Avalanche with the hard 3 piece cover.I purchased it 8 months ago and the cover has always leaked.I went to my local dealer and purchased all 4 cover seals and 2 side gutters. I took the covers off,cleaned the heck out of them and installed the new seals.Went out and installed the new gutters,then reinstalled the 3 sections.I thought for sure I fixxed the problem.It rained over night (1st night) and I went out this morning to find my covers still leaked.Has anyone else had this problem? Any ideas how to fix this problem? I am out of ideas, cant see any other parts to replace to fix this problems. HELP!!!!!!!           JOHN