I'm reviving this thread because I just had the same problem with my Model 66.
I was taking the lock out, I had previously removed only the "flag", but not the remaining parts. I plan on ordering one of BullesEye Smith's now famous plugs.

I had the same "hitch" upon re-assembly. I found this thread and re-positioned the main spring several times to insure it was lined up.

The action would function perfectly without the hammer block installed, even with the sideplate back on and the screws tightened.
Of course, in the process of all of this I had to re-install the hand spring twice, a chore I could live without.

I finally took a Dremel and lightly polished the surfaces of the hammer block, especially at the bottom where it travels around in the side plate channel.
That solved the problem. I have had this gun apart many times and never had this problem. The only thing I can think of is that maybe the rebound slide was some how moving just a hair more than it ever did and this was cramming the hammer block against the sideplate channel.

I was up until 3 AM messing with this thing. Now I'm wondering if maybe I didn't seat the hand spring into the blind hole and possibly the trigger lever isn't seated properly and causing a little extra throw on the rebound slide.

Also, does anyone know if the rebound slide is reversible, meaning is there any difference in which end you put in first?

ETA: Indeed, it was the hand spring. It was not seated correctly and was causing the rebound slide to push the hammer block too far to the rear.
Oh well, an extra drill on hand spring placement and now I have a shiny trigger block.