Ah, the age-old problem. Basically, you gotta face reality, you can't put ten pounds of "stuff" in a five pound bag! You have to either get a larger bag or less stuff, there's no magic tricks available here.
More to the point, what grade are you using, and what kind of RR are you trying to model? Generally, try to keep the grade under 2% to avoid the problem you're having. If you need to determine more specifically how steep a grade your equipment can handle, use a "test track" with an adjustable grade (nothing fancy, a 1X4 with a straight piece of flex track fastened on it, and a few books to raise/lower one end of the 1X4).
Thats how you figure out how much stuff you got. To make a big enough bag, you need to make the approaches longer. Can you add a (dare I mention it) a helix? Or add a few zig-zags here and there?
Finally, make sure there isn't an abrupt change from one grade to another or from grade to level. Looking at the grade from the side, you need to round off the corners. Just visualize a train starting to go on to or off of a grade, and what happens at the center of the cars and at the couplers, and you can see why. (Unfortunately, this makes the length required even longer.)
Bill K
p.s. BTW, welcome to the Gauge!