What's the go with Manfrotto Tripods? My legs have been slipping gradually over time on my 190xproB when pressure such as a camera is put upon it. Annoying as you can imagine - set the shot up and then one of the legs starts to contract! Is this a fault, or is this normal over time, and can something be done to fix it? I have tried looking at the screws but a normal screwdriver doesn't fit in it. Thanks in advance for your help and insights.