problem with a bosch fridge?

I have a relatively new Bosch fridge freezer. The fridge is one of those "self-defrosting"types. As the ice at the back of the unit thaws it trickles down the back fridge and out through a small hole where it re-circulates. The problem is the hole keeps blocking and the water has nowhere to go except the bottom of the fridge where it ends up on top of food. It's a mess clearing this water out on a regular basis. What is causing the hole to block? I've tried clearing it but maybe its air trapped which causes the problem?