Player is sticking to walls

Ok Guys,

My game is essentially like any maze game ( pac-man ). I modeled the walls with invisible 2Dstaticsprites, set the collision to FULL and the response to CLAMP.

This is keeping me awake :

Sometimes, when coliding with wall ( say left wall ), the player gets sticked to it. He may move up or down, but not right. I can only move right if I move (up or down) past the wall. The ASCII art bellow shows a case :

+--------------
     | 
     |P                
     |    +---------+
     |    |         |
     |    +---------+
     |      
-----+

In the example, the player (P) gets stuck on the left wall and cannot proceed right, even though he could

The player's collision polygon is also a box ( but not the full sprite )

TIA