Two things. As MarkT mentioned, the MCP4131 has a limit of 5 volts applied to the pot terminals. Two, you have it wired wrong, it should be:
S0 (gnd) -> POB (7)
S1 (+10 ref) -> POA (5)
S2 (spd ref) -> POW (6)
So long as you are only running the drive in one direction, you might be able to make the MCP4131 work but you'll probably need replacements as the one(s) you've used may be damaged.
The trick is to see if the drive will achieve the required speed with only 5 volts input. You can test that using the standard 10k pot that appears to come with the drive. Set it so you measure 5 volts on terminals S0 and S2. Then, see if you can get the drive to maximum speed required by increasing the drives max speed pot.
If so, insert a 10k resistor in series from S1 (+10v ref) to pin 5 (POA) of the MCP4121. This will limit the maximum voltage applied to the pot to 5 volts. That should allow it to work so long as you compensate for the 5 volt maximum input reference with the max speed pot of the drive.