07 600 wont go into rev.

Gotta press the button for at least 5 sec to get it into or out of high elevation mode.

at what altitude is the sled run? there is a different mode between high and lower altitude on the carbed models. look at how fast the rev light flashes. if you hold the rev button for something like 5 seconds it will switch to the other mode. high alt the rev light flashes faster. it changes the timing.

reverse

Those guys are correct altitude can effect the PERC system. It is also possible that your drive belt is out of adjustment. If the belt is to tight it can put unwanted tension on the crankshaft not allowing it to go in reverse. To test this theory, open the sheeves on the secondary, allow the belt to be very loose, try to put it in reverse and if all goes away then problem solved, adjust the belt. One other thing is possible low compression. Although your sled is running and obviously your riding right now, it is always a possibility.