X2 on what Jordan said. Get under the back of the boat and take a look inside the intake. Make sure there is nothing in or blocking the front of the impeller. You might end up having to separate the bowl from the suction part of the jet. This will let you see the inside of the bowl and impeller.
This has happened to me twice. The first time, I found a rock in one of the vanes of my bowl. It was about 1 1/2" in diameter and 3" long. I have no idea how it got past the impeller. It did not make a noticeable difference in how it ran. I just happened to see the back of it while looking up the back of the nozzle.
The second time, I was idling out of a shallow marina, heard it scrape the bottom, and shut it off. I floated to a deeper area and started it up and felt the vibration and it wouldn't go. Put it on the trailer. I didn't see anything. Took the jet apart, nothing. I put it back together an it ran fine.
Sent from my iPhone using SoCal Jet Boats