Where do you have your footstraps (forward, middle, to the rear).
Your 7.2 m2 rig should have you flying along in 15 -17 knots at 85 Kg.
Do you have the center board down or up..... and, what fin are you using
in the back of the board?
Where do you have your mast foot placed in the slot...front.... middle...rear?
The Rio M is pretty easy to get on plane, so I'm guessing you may have some
sort of tuning issue.
What year model is your Rio M? Is it start based (wide and short 2007 & earlier) or is it the later longer narrower Rio M 2008 & 2009.
Hope this helps,