Why am I getting no power to the branch circuits with a new sub-panel?
Start by shutting off the breaker in the main panel that feeds the subpanel.  

Using a multimeter set to check continuity, test the continuity through the main breaker in the subpanel. It's possible that the main breaker in the panel is bad, and needs to be replaced. The problem is, it's usually cheaper to buy a whole new panel then to get a replacement main breaker.  If this is a new panel, and the breaker tests bad. I'd contact the place you purchased it from, and ask them what your options are.

While the power is still off to the subpanel. 

Check all the connections to make sure they are tight.  

Check the continuity from the bus bar through each branch circuit breaker. 

Make sure the main breaker in the subpanel is fully closed. If you turn it to the ON position and it has a little wiggle to the handle, this might mean the breaker is not actually setting when you turn it on. In the ON position, the handle should held tightly in place. If the breaker will not set, it means the breaker is bad and needs to be replaced.