water treatment/perculation basin

Can you include a sketch of your system so that everyone here can have a better idea what's going on? Also, what's the size of your grease trap & its outlet pipe and your wastewater flow rate? Your flow rate could be too high for the water to drain away effectively.

You also mentioned gravel. A grease trap usually has oleophilic baffle plates or a series of separate chambers to separate and trap the oil, not gravel. Are you using a multimedia filter as a grease trap? If so, the film is almost certainly the cause of your problem as oil coating the sand granules will in turn cause fine suspended solids to adhere to them, eventually clogging your system. If this is indeed the case, you'll need to backwash your filter, and if it's too badly clogged, you'll need to replace your medium.

A drawing would be nice.

Is the water hot? If so how hot?