My XFOb (Fob) will not pair with my MOB+ Hub.

This could be due to a couple issues. 

1. A weak battery- The CR 2032 batteries are known for having different geometries. When changing batteries do the following. While holding down on the contact point, gently bend the tabs out a little to ensure they will make solid contact with the new battery. There are 4 tabs on the bottom and one on the side.
2. Gen 2 Xfobs will not pair with Gen 1 hubs.