My experience with stainless steel reels is that the film doesn't always slip naturally into the space between adjacent turns of the spiral. I think the reason for this is that the spiral is wound from round wire, and the film slips over the round edge and skips to the adjacent space. I have some old Durst plastic reels in which the spirals are molded with a square cross section, and the steeper walls of the square side retains the film better.

You do need to cup the film slightly, especially to get it started in the reel. Once the film has been attached to the center of the reel and starts into the groove, it "should" flow naturally. But I find that cupping the film about an inch or so before it enters the outer diameter of the reel makes things go smoother.

One of the common problems I have seen is that if you cup the film too much, you will get half-moon-shaped shadows on the developed film. Go easy!

Someone suggested Hewes reels. Clearly they are the best. But others will work also. But be careful about buying used reels - if reels are dropped (even good Hewes reels), they will distort slightly. It is very difficult to load a reel if the two faces are not absolutely parallel. So if you are having a consistent problem, check your reels to see if they have become bent.