Try these ideas
You don't give much to work with so try these ideas. I went through this with a 9.9 on my old 27. 1. Adjust the shaft one click so that the propeller is angled further from the stern. This gives more room between the prop and the rudder.2. Try moving in reverse at different speeds. See which one gets you going.3. Make sure the tiller is centered before you start.4. Make sure reverse is actually engaged and works.