Gullies are designed as a trap: they catch debris so it does not enter the drain. That only works if the debris is removed. Left in place, it compacts and eventually blocks the outlet.
The job takes minutes with gloves, a trowel and a hose. Scoop the silt out, flush the gully and check the water drains away freely.
Commercial premises and car parks should be on a scheduled programme with documented visits — it is much cheaper than reactive flooding call-outs.