It's been a long time since I had a m110.

Did you take the top half of head off (with the cams)?
If that is off it is easier to see if you misses a bolt.

There is also an oil return line in the back (under the intake manifold)

Maybe a pin of the chain guides is still in.

If everything is clear you may have to use some force, gaskets do stick after a while.

Rob