Another more unusual issue is the shell latches. I had the exact same problem you are facing. I changed the follower and spring and it didn't fix it. I then took out my trigger group and noticed the left shell latch fell out.
The shell latches are staked into the receiver and are not meant to be removed or fall out. Remington 870 shell latches are supposed to be staked three times on the top and bottom. My Grizzly latches are only staked once on top and bottom.
I bought a new Reminton 870 left side shell latch and installed it. Functions flawlessly now. I did notice the latch tip looked different from my Grizzly one, so perhaps my old Grizzly one was damaged.