It's hard to diagnose. First off, the clarifier may well have gummed up your sand, in which case the cheapest and easiest solution is replacement. Teapot's right about the channeling.
Other things are that your filter may be too small for your pump's horsepower, or you have damaged laterals (internals of the filter).

Sand doesn't wear out, and, in fact, a well-tended sand filter tends to filter BETTER as the sand gets older. The hard-edged points on "new sand" get worn down and it packs closer trapping more dirt. My sand is in its 13th season and just getting better and better.

But without knowing the size and capacity of your pool, filter and pump, it's hard to diagnose.