Did you set the current on the DRV8825.
A current controlled driver is a poor choice if you just use it as a H-bridge (full voltage all the time).
A 5volt motor could be a better match for a current controlled driver on a 12/18/24volt supply.
I could get 16RPM (unloaded) with basic ramp-up/down, without modification of the 12volt motor.
With a TPIC6B595, used like the standard/supplied ULN driver.
Not sure why you want to go the DRV8825 way.
Leo..