sounds like you have some magnetism in the balls. I replaced my balls and the new ones I used had that issue in my HRC. Turns out they were slightly magnetized. It was hard to tell but out of the machine you could see hey were sticking. I suggest taking them out and seeing if they are magnetized. You say "sometimes" so it may just be one or two of the balls.

When I put in new balls (Ball Baron Balls) they were magnetized when they got to me. I actually put my old balls back in because they were fine. Problem solved.

It is really easy to see if they are magnetized. Take them out and put them on the flat floor and move them around and see if they stick to each other at all.