This Lee online help video covers this exact problem. Note how hard the fellow pushes on the second pass. He advises 25 lb on the first pass and pushes a good deal harder when he finds the bullet slipping into the case afterward.
Note also that the pressure numbers in pounds only work for the Challenger press with its particular compound leverage. If you have some other press, the force will be more or less depending on what mechanical advantage the press has? The Lee Classic Cast press, for example, has higher leverage and makes this job easier. So, you have some trial and error work to do.
In addition to the above, I find with some brass it can be useful to pay attention to two more things. One is that some cases have slightly off-axis heads or necks, not to mention uneven necks. I always rotate the case 180° and give it a second push to try to even the applied forces out.
A second point is, while the collet dies work the necks less than standard dies (because they never over-resize and expand, as standard dies do), annealing necks so they don't tend to spring back can be helpful to minimizing the sizing force, especially on the first pass with previously reloaded cases.
Finally, once in a long while, no matter what you do, the brass and press you have just won't go. In these instances, you may either send the die back to Lee with some of your cases so they can adjust the mandrel diameter, or you can do it yourself with a variable speed drill and some 400 grit wet/dry sandpaper moistened with mineral oil. You just slide the paper back and forth along the mandrel as it spins in the drill. Don't take off more than a thousandth at a time. Half is better. Make frequent measurements to check progress if you do this. If you polish the mandrel with crocus cloth afterward, the smoother surface will be less prone to rusting later.