Loss of acceleration

Hi all I'm a fairly new boat owner although I have decent mechanical ability I don't know all the working of an outboard. I noticed last weekend while out on my 16 ft pro v bass tracker with 40 hp mercury force just me and a buddy and his son boat wouldn't come up on plane. Weight had never been an issue before and seemed our speed was not up to par. This weekend we went on bigger lake with my buddy and his wife and while we were loaded more this time we were able to get on plane initially with 3 full size adults. Ran 17 mph. We stopped to fish then went to move again this time 8 mph WOT all the way back to dock. Motor sounded fine but just didn't want to move very fast. After they unloaded while I was waiting for them to back trailer down and again I was able to get up on plane this time but just with myself in it. Does this sound like partially spun prop? Or should I be looking elsewhere?

Is speed increasing with the increase of rpms? Are you reaching WOT with engine??

Remove the prop, mark the hub and reinstall.
Go for a ride and see if the line moves when it does it again.

Buzz, let us know whats happens.