Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I'm sorry to say that, if by 'the belt' you mean, the parts of the printer's scanning apparatus that move under the glass sheet (including the scanner bar), then either you have a defective scanner (defects in electronics, or in position sensing for the belt, might explain it), or else perhaps, your HP machine is somehow being directed to scan through the ADF feeder, or top feeder, but your original is on the glass sheet. (This last could happen, perhaps, if a paper presence sensor element in the ADF feeder on top of the printer, were either registering document presence falsely, or else if if were fouled with debris, paper scraps, etc.
I would advise you to examine the top feeder, by opening it up and looking for paper scraps, fluff, debris, etc.
Let me know if you need any further advice, if there are details I can help with, etc. Thanks.