the first site talks about even though the paper jamb is fixed, that the parts or rollers/grippers might be worn or dirty or misaligned.

If you can take a light and inspect all those parts as best you can , you might find the cause.

one HP i got at a thrift store just had a little guide flipped the wrong way- I just noticed it and flipped it back and it worked fine.

You might also go to the HP website and see if there is an updated driver or software for it.

how old is it?
sometimes they just wear out & break too.