Not sure why you think I don't agree, we have just said the same thing. I agree that if it gets stacked up on top they will stop it. I, however, have never seen that happen, but, I can see that it might. I also understand that some people might have a problem with mobility causing a stop, I know that, but, I have personally never seen it. I have, however, in recent years, seen it not operating a couple of times with no apparent reason other then it must have been broken. Keep in mind I'm not there everyday so it could be more, that is the only time I have seen it not working was in recent years. The previous 31 or so years, nothing.
We are saying the same thing in different words. You mentioned one possibility in your original post and I just added some other reasons.