Kaz you might be on to something. I used to fly 172's and some models have a placards on the dash warning about slips with flaps extended. Today, I was out with our club chief CFI and demonstrated the maneuver at altitude, and he thought that the tail surfaces were getting blanked by the flaps. It didn't really STALL this time though, but I did not hold it long after I felt the controls get mushy. Once bitten - twice shy. I'm going to try a good full rudder slip without flaps next flight to build my confidence back up. If I survive that - then maybe again progressing to full flaps.