Here's what I did, in order:
1. Created a boulder sprite.
2. Created a "boulder hitbox" sprite and stuck it right on top of boulder
3. Set the boulder hitbox sprite to Qualifier 4 and unchecked "Visible on start"
4. Event editor: "When player collides with Group.4, player stop"
5. Tested the boulder, everything works as intended
6. Copy-pasted a bunch of the boulder+boulder hitbox and stuck it everywhere
But then the player glides through the other boulders. The original still works, but none of the copy-pastes do. They're all still part of Group.4 and I deslected "Destroy if too far from frame" and also "Deactivate if too far from window". Any ideas?