As you apparently know, the Magazine Follower is holding the Slide open until the Magazine drops down and the Magazine Safety is timed in to continue to hold the Slide open. So in the condition you mention, to remove the Magazine with the Slide open, you have to overcome the pressure of the Recoil Spring. If the Recoil Spring is too heavy, i.e., from a .380 HSc, it will be very difficult to remove the Magazine with the Slide open.
Other than that, a burr on the Follower or Slide below the Firing Pin port could cause the Magazine to hang up when trying to eject it. Or anything else rough in the top of the Magazine Well that would hold it with all the pressure of the Recoil Spring pushing the Magazine forward.
Hope that gives you some places to investigate. Let me know how you make out.