Could be any of several reasons.
1) Is the printer under warranty? If so, call HP and let them service it. No fuss, no muss.
2) Perhaps you are using non-HP ink, and either the carts were filled wrong, or they aren't telling the printer that they're empty, so it's printing incorrectly.
It's hard to diagnose this without more details, so before I try, do this:
A) Tell us, exact printer model,
B) Tell us if you're using HP or non-HP cartridges or toner
C) Tell us if it's under warranty
D) Take a photo of what it is printing, and attach it to your question as a comment if necessary.
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