To reiterate: Did you earn the Plat in the game you got your first trophy?
My first trophy, as seen below, is The Graduate from Ferrari Challenge Trofeo Pirelli. The game was patched to include trophies as it was released just about the time Sony implemented that feature. It took me awhile before I got back to the game but I eventually earned the Platinum in it, THREE YEARS. I just wonder if anyone else had a similar experience with earning the Platinum for the game they got their first PS3 trophy from.