I am perplexed why you had to remove the heater box. The bolt should slide out easily with it installed.
As Bob said, make sure you only have one washer installed and that the shaft is free of burrs, etc. Make sure you put a little lube on the shaft to make it easier to get it off later on. I actually fabricated a spacer washer that was about half the thickness of the Vans provided washers. I needed this additional amount installed after 300 hours or so. I was able to compress the assembly by by having someone under the tail using his back and someone at the prop pulling down. The guy putting his weight on the prop flange weighed over 200lbs and probably had 3/4 of his weight on it pulling down. As bob stated, make sure the plate is seated properly on the mount. After painting the plate, I used a bead of high temp RTV to hold the plate in place.
RV-10 - 950 hrs
First Flight Oct 2012
Aviation Partners, LLC