As suggested above, the first thing I would check for is a spun hub. If you're not familiar what a prop hub is, it's a insert in the prop. When there's an impact the hub breaks free (or spins) within in the prop so the sudden impact of something stopping the prop doesn't ruin the prop shaft and or your gears...
The symptoms of a spun hub is an engine that races but does not get up to speed... The hub breaks free and spins within the prop instead of spinning the prop. It's kind of like spinning your car tires on ice... The engine will race, but you won't go anywhere...
This video will help to explain
https://www.youtube.com/watch?v=d7Lz7cI-srQ
I were you, I would take the prop off the engine and take it to a reputable prop shop and have them see if the hub is spun... If it is they can re hub it for you and it will likely be around $100 or less to re hub it.
If that's not it, you have something else going on.
