Did you say the timing marks on the cam and crank are both at 12:00?
The cam mark should be at 6:00 and the crank gear mark at 12:00. they should be pointing at each other.
When you install the cam, you match the marks up, crank at 12, cam at 6. This is to line it up, but it's actually TDC for #6. To get to to #1 TDC, you rotate the crank one full turn, which rotates the cam 1/2 turn, and puts #1 at TDC (both marks at 12:00).