How can I stop my toilet from blocking?

I have a new Villeroy & Boch toilet, and every 3 or 4 weeks the u bend gets completely blocked up and when it flushes the water goes down very slowly indicating a blockage. I have to undo the access side pipe at the back of the loo, and poke the blockage with a stick to clear it, then close access , flush and repeat until loo paper etc is cleared through. It is then fine for 3 or 4 weeks when it happens again. It is only used by children and none of other toilets in the house have ever been blocked, nor that one until new bathroom installed. Plumber has checked and no obvious problem. help!