Your ATP is an 04 model? IT has the new style diff in it with the "plastic" roll cage that engages the AWD. I have seen the later model machines (with this setup) have problems engaging and disengaging whenever they seem to feel like it. Im not saying that the cage is the problem, but it stands a good chance of it.
What kind of fulid did you fill the front diff with? If you used gear oil, drain it out, make sure its clean, and refill it with some Polaris hub fluid. I seen that work on one guys machine. They are supposed to have hub oil in them instead of gear oil because of the AWD system.
-Will
1985 Honda ATC 350X
2003 Yamaha Banshee
2003 Polaris Sportsman 500
There are 3 kinds of people in this world, Those who make things happen, those who watch things happen and those who wonder what the hell happend.