I had issues with button#4 as well. Needed to be pressed all the way down in order to toggle consistently. This was not the behavior with the other buttons. Had it replaced, and now button#5 is acting the same way as the previous unit. Unfortunately, I am in another continent and returning/exchanging units is very expensive.
recived my new unit last week
working flowless so far...
try to replace button#5...
sad,to know,its more peple with same problem...