393 Throttle Problem

All,
Hoping for some direction on my query.
2002 Model but extremely well maintained yacht.
Engine (55HP Volvo Penta) serviced 3 months ago - clean bill of health.
Out on the water yesterday and moved throttle into forward gear.
Revs came up but did not engage in forward.
Moved throttle back to neutral and tried it again - this time gear engaged and off we went!
I then slowed her down to a stop, throttle in neutral and i put her into reverse.
Again high revs but did not engage.
Back to Neutral and then back into reverse and she dug in no problem and we had way.
Anyone out there seen this sort of thing before and if so where source of problem could be?
best wishes