improper headspace causing the barrel not to lock up fully (each frame could have different measurements and your brass could be too long), or your bolts are not properly engaged, and yes some frames are different, or maybe the trigger guard return spring being misadjusted preventing proper lock up. Check the area where the locking bolts from the barrel lock up on the frame, look for obvious damage such as rounding off of the edge in this area. Also ensure your interlock safety is working properly (I'm assuming this is a newer frame). With a barrel not on the frame you should not be able to make the hammer touch the firing pin. If you can then your interlock is not keeping the frame from firing without the barrel being properly locked up. DO NOT FIRE A FRAME IN THIS CONDITION. take it apart and clean it, you can go back to a great reply Gramps posted about a gun that wouldn't fire and read the reason to do this and how (don't just spray it with cleaner and call it done) it's under the "new user" topic. Although he's persona non grata around here, Bellm has a good article on his web site about this problem.
Helicopter Bill