There could be possible ...

There could be possible biofilm formation which is causing mud-balling and wormhole channeling. These bacterial colonies can develop quite rapidly given the right temperature and food source.

If you are looking for a more reliable filtration without clogging, I suggest switching your sand to an Activated Filter Media. Biofilm cannot grow on this type of media and performance is up to 80% better than sand. Thus, chlorine consumption is significantly reduced. 

Please email me at andrew@waterandoilsolutions.com.au if you are interested in investigating further.