Ok , so the sense resistor is 0.05 ohms , and with 300mv across it, means that there's 6 amps of current flowing through the pump diode( 50mv per amp). Still less than the 8 amps that the 5v supply is feeding into the circuit.
For an idea of how this part of the circuit works, look at:
http://www.repairfaq.org/sam/glpdrv1.gif
Much smaller scale, but the output stage compares. R9 & R10 are the current sense resistors, one end attached to the neg rail, the anode (+) of the pump diode attached to "ground", the opamp (u1A) turns on the drive transistor(Q1) to allow current to flow through the diode. The opamp U1B scales the voltage read across the sense resistors, opamp U1A then compares this value with a reference voltage (pin 3) in order to control the amount of "on-ness" of the transistor. Your driver is a lot more complex, but that's the principal of it.
I think that now you've worked out that the power supply is OK, and you're feeding it a good modulation signal, If it's still not working, then it's time to talk to the manufacturer , because I'm running out of ideas.
B.