If you can observe the ball bearings, you should see that when you wind the reel toward you, the ball bearing rides up allowing it to slip pass the film, then when you wind the reel away from you, the ball bearing jams or grabs the edge of the film pushing it forward.
Each reel is always going in the opposite direction so at pass/grab action keeps going back and forth, like a person walking. So, you are trying to walk the film on the reel.
Some older models had no ball bearings and the user would provide this function with their thumbs.
You normally start the process by sliding the film onto the reel, pass the ball bearings as far as you can, usually several inches, then start the walk.
Also, clip the corners off the leading edge so they don't jam into the plastic. As others stated, reels must be totally clean & dry.
Hope this helps.