It's a mag / mag spring issue. See part in bold below...
Having 8rds in a standard length tube like those that come with most 1911s will have more spring tension and the cartridges have less "breathing room" That "feels like something hitting" is in fact the top cartridge making contact with the bottom of the slide, the rounds don't have alot of room to further compress into the magazine so it feels stiff. A good firm slap should get the magazine to go in.
As for with the slide pulled to the rear, same thing, the slide has to overcome that high tension when stripping the top cartridge from the magazine. Load and unload the mags a couple dozen times and the springs will start breaking in.
Switching to a magazine with a longer tube like the Wilson ETM or Tripp Cobra Mag should eliminate the problem all together.