Always go for the obvious first, although don't be suckered into excluding other things. I have a 35' sailing boat, and one year had the single lever (throttle and forward/reverse combined) replaced. First trip out, just to "try" it out, all fine, went about 300 yards, then decided to engage astern. Nothing but engine revs... Never mind, we can get back onto the mooring in ahead only. Engage ahead, only engine revs... I assumed that there was a problem with the new control as first point, but when I went below to look, the propeller shaft had dropped out of the engine coupling. Entire coincidence.
2018 Bonnie Speedmaster Jet Black with Highway pack (less bags) and rider footboards