motor wont turn over?

ok I have a very dumb problem but I can't figure it out. I have a 360 that will not turn over. I put a mini starter on it the other day, today I put a set of plugs in it and tried to turn it over to make sure the rockers were getting oil. well, it would not turn over, so I put a different starter on it thinking that the starter was bad. both starters are good, it turns over like crazy with the plugs out and I have the valve covers off so I can see the valves are opening and closing does this sound like a timing chain maybe? I just put the heads back on the engine and just can't seem to figure it out. The only thing I can figure is the timing chain is not right and the exaust is not able to exit the cylinder and to much compression is building up in there? any help or suggestions would be great.