All of the above.
The bolt hold open should be cleaned of grease or Cosmoline and left with a light oil. A spray solvent/oil can be useful in cleaning it all out.
When it's clean and functioning properly it should rise up rapidly upon releasing the magazine follower and drop down easily upon depressing the magazine follower. The spring is not super strong, but it easily causes the bolt hold open to rise quickly if it's clean and properly lubricated.
Correct stripper clips in good condition cannot engage the bolt hold open, nor can the rear of the cartridges loaded from properly functioning clips.
The only other remote possibility I can think of at the moment is that the bottom of the bolt in the front was ground down by Bubba for some unknown (and dangerous) reason. That'd be very bad and would be a
stop using this bolt immediately situation.
The front of the bolt should have about 4mm (in the center) to 7mm (on the edges) of a solid lip below the bolt face where the head of the cartridge rests (just eyeballing the dimensions, I didn't get out the micrometer). This will solidly engage the bolt hold open and keep it "up" due to pressure/friction, until the magazine has a cartridge loaded into it and the bolt carrier is bumped back to release the bolt. The bolt hold open when engaged extends all the way up to about 1 mm below the bolt face, i.e., that solid lip has a lot of surface area interacting with the back surface of the bolt hold open.
P.S. Welcome to The SKS Files. Glad to have you!