I had that problem. I ended up running a plumber's snake through while running a good stream of water to chase the stuff I loosened up out.

And ever since then, I'm really careful to sweep the patio, not just use the hose to wash everything into the drain. The pipe is too small and there isn't enough pitch to keep it all moving.
 
some drains have a top piece that can be removed some are solid ( we have had to cut tops off to clean some thats a worst case thing tho) how old is it and do you know the maker ?
 
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