You can back flush the classics, they actually recommend it.
Did you replace the floss? Depending on how long you wait to clean it a back flush may not be enough, sponges get real dirty.
Take all the media out and give it a good cleaning of its been a while.
Backflush with it disassembled though. The only thing I back flush is the canister exist tube without the media basket. I rinse everything from the media basket in the aged tank water that I am removing. Even if they recommend back flushing the way your doing it, I wouldn't.
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Those 2211s should really not be treated like a full size canister, despite their instructions. They easily lose flow, mostly due to the very small diameter of pads.
If you backflush it, all the accumulated gunk will be 'released' and quite a bit of it will end up in the floss pad - resulting in a clogged canister. There really is no way around taking it apart every time it needs to be cleaned.