Likely the pre-filter is clogged. Change that. It's called the "Secondary" filter.

If it clogs up again shortly after, you have a bacteria issue that needs to be remedied with a Biocide or tank removed and cleaned. Also, the tank screen will be dirty as well.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k