Mick
Thanks for the manuals. I see now what type of kit you have.
The motor speed is proportional to the (effective) armature voltage applied to it. As you have applied both 1.5v and 9.5V and these resulted in similar speeds, something is clearly wrong and I would start by checking the bus voltage, as this is the maximum that can possibly be provided by the bridge output circuit of the drive. I'm not sure which motor you have but the specs give the voltage factors per radian per second so this should enable you to estimate the required voltage for the maximum speed you need to operate at.
The drives appear to be designed for a voltage input, so we are OK in that respect. They either implement a simple (switched mode) amplifier or a velocity loop if you have the tacho installed. Either way, they convert input command voltage to motor speed, which is what we need.
Sorry if this seems tedious, but we need to proceed logically one step at a time to try to identify the issue. If the bus voltage seems OK then something within the drive must be limiting its performance.
As Craig says, advise us if you have the tacho installed.
Allan