The USB-6501 power is dependent on the USB bus voltage. This may vary from one motherboard manufacturer to another, but the USB 2.0 specifications state that the valid range is from 4.4-5.25 VDC.
To solve this issue, you can incorporate an external resistor between the 5 V bus and the digital output line that will allow the digital output line to be pulled up to the 5 V rail. This user supplied resistor allows the USB-6501 to drive up to 8.5 mA of current to a load. As seen in the diagram below, the external resistor is in parallel with the 4.7k on-board resistor.
Instructions on how to select the appropriate value for the external resistor is included in the NI USB-6501 User Guide
under the Digital I/O section.