Under Sink Water Filter

I have a GE Smartwater under-sink filter that’s about two years old. The last time I replaced the filter cartridges, the flow of water was reduced to a trickle. Thinking the new cartridges might be defective, I tried putting the old cartridges back on, but it still barely worked.
The water flows at full pressure for a fraction of a second, then narrows to a trickle. I actually get a little more water by keeping the filter supply line’s shut-off valve at about halfway, but it still takes forever to fill up a glass. There are no leaks and the filter cartridges are on properly.
Is there a way to fix this, or is the filter unit itself worn out?

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