Sand filter failure

[Old Eagle]

Old Eagle, new sand is a hoax, it turns out to be just as old as the old sand well +/- a million years or two.
Sand doesn't wear out in sand filters because it's stationary for most of it's working life, it get dirty, bio fouled stuck together and therefore channeled.
The channels allow free un filters passage which is your problem sounds like.
The cleaner you used may not have done the whole job and I have noticed just how bad domestic filters can backwash. In one case we removed the media into dustbins and thoroughly cleaned it before putting the same media back which is now once again perfect.

I am willing to try pulling the sand out and cleaning, but what did you do to clean it?