I use some rubber snap-on tips from Black Diamond. They’re sold as an accessory to several of their poles, and they look a lot like the ones in the Amazon link above. I like the rounded tips because then they don’t have to be a certain orientation to work.
With respect to missing the belt, how wide is your treadmill belt? Mine is 22″, and I’ve never had an issue.
In addition to belt width, check that you’re not “A-framing” your poles (narrow at your shoulders and flared at your feet). That might be the issue as well. Make sure that your pole strikes are vertical, both for hitting the belt and generating the most downward force.