Belt slipping?

I bought a Sole F63 (2013) used recently, it only has 72 hours of use on it. I noticed right away that when I was running and I would stomp my foot down that it felt like the belt slipped. I lubricated the belt with the prescribed silicone, and tightened it. Tested it, tightened it, tested it, tightened it. Every time I think I've got it tight enough then at some point during a run I get that sick unstable feeling of the belt slipping. I'm worried about tightening too far. I can slip my hand under the belt to the middle of the belt with some force, but I don't imagine that I can tighten it much more before I have to use force to slip my hand under. Should I just keep incrementally tightening until I no longer get that slipping feeling every now and then? Or should I be doing something different?