If you have ever driven a car, and tried to start out in 3,4,or 5th. gear, you would know why your rider won't go up a steep yard in 3,4,5th gear.
About the deck height settings: If you have the blades higher than the top of the grass blades---yoo-hoo! It ain't gonna cut much grass!
My old Granny had a saying: "Buy them shoes, send them to school, and they eat the books!!"
My experience with most any powered mower, is: The manufacturers of the machine have spent many "moons" designing and building the machine. Way i feel about it---don't mess with a good design.
As for your machine: re: the drive disc area. If you don't know anything about it--don't mess with it., don't allow it to get wet! And, don't expect any powered machine, a Snapper, Craftsman, Toro, or any other, to climb hills like an Army tank!

Thanks Rusty for the comments and the tinge of sarcasm :)> I am going through the mower to see if it needs anything, greased the fittings etc.

I did run in to one item. I pulled the right rear wheel due to the amount of grease on the hub. (the right side does not have a grease fitting since it is lubricated through the rear end fluid). I found a thin nylon rope wrapped around the axle with a lot of grease on it. I cleaned everything up, checked the play in the axle which seemed ok, and reinstalled the wheel. Next thing will be to check the rear lube level as it appeared that there was a lot which had leaked out.

I guess I will find out if i have a bad seal if it continues to leak over time.

Kenstl: the most common "no drive" or slipping problem is wear on the "rubber tire" disc,this is the part designed to wear out.be sure to lube the drivers L/H side bearing,the axle can wear through the bushing into the fender and ruin the axle(expensive) also check the grease in the transmission keep it even with the bottom hole with mower on end.There is a rubber boot on the R/H side on the outside of the bearing if this was damaged it's an easy fix,except you must remove the wheel hub which usually not too bad on the R/H,but the L/H is usually a bear.Good luck with your RER.