I had a brand new belt on mine and it was a tight fit to begin with but was slipping from day one. Since then I hve had to tighten it 4 times - and its only been on there about 4 months, if that!!!
They slip very easily and need to be very tight. You will find though that they tend to slip in wet weather regardless. The rain/water does it, so just be aware of that and dont tighten in wet weather
Dont you just love the power/responsiveness you get back into it though when you tighten it? Its like a brand new toy