The PTFE hotend isolator part has a tendency to deform over time, especially if you print with "hot" or temperature conductive materials. Once it does, it will cause blockages, and it should be replaced.
I think you can still find replacement PTFE parts from chinese manufacturers on aliexpress.
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Umo_Alex 4
I've been able to clean things up a few times by heating it up with a hot air gun and blowing compressed air through it...
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