Did you replace the filter, if so, put the old one back on, and try that,, wouldn't be the first time I had a new, defective filter, or new, plugged muffler, but that's another story, just sounds like a pressure prob..
Make sure the manual valve that the shift linkage moves on the valve body is correctly engaged to the linkage and moves when you move the shifter.
It's easy to miss when going back together.