car starting problems fuel pump maybe? but.....

ok when i try to crank my car it wont start like its out of gas. i unpluged the fuel line from my block and tried to crank it to make sure gas was getting through but it wasn't. so my first thought is that its my fuel pump. now here is where it gets weird. i can push start my car because its a manuel and its starts, runs fine, and doesn't stall out or anything so obviously the fuel pump works once the engine gets started. so what could this be?