Sled can't get up the trailer!

So last night I put the sleds on the trailer. The Arctic Cat couldn't make it up. It seemed like the belt was slipping. This is the first time it has ever been under a load. I know the last time the sled was running it was just fine (I seen my neighbor riding it in the yard). After me and my son got it up on the trailer. I checked the belt and it looked like there was an oily substance where the belt rides on the primary clutch. The belt itself looks good, no cracks, worn spots or anything like that. I'm taking them to my buddy's place for some test riding and some fun. Looks like I have something to fine tune here. I do have an extra belt if o need it. Also, the clutches both move when the track is off the ground, so I don't think there is a clutch issue, but this is also the first time I have ever had a snowmobile. So this is a learning season for me!

Thanks

Sled can't get up the trailer!