Maserati Mistral Crankshaft Rotation Problem

I have just finished an overhaul of the cylinder head on my 4-liter Mistral engine including replacement of the valves, valve springs and timing chain. With the head reinstalled with a new head gasket, I am now unable to rotate the crankshaft more than 20 degrees from TDC, despite the fact the hash marks on the front cam towers align with the lines on the cams (at TDC). The timing chain rotates 180 degrees easily from the crankshaft when the cams are disengaged. With the No. 1 piston slightly lowered, I am able to rotate the cams (one at a time to avoid valve interference. I want to resist the temptation to apply force to the crankshaft and risk bending the valves. With the plugs removed there should logically be no ideas why rotating the crank should be that difficult. Any help would be sincerely appreciated.