There could be many reasons why this is happening, but for a start, I would suggest the 3 items below.
1. Hook the CAN ground to your system. There is 1 pin into the 9 pin connector for ground, it should be hooked to your CAN ground to eliminate interference.
2. Check to see if your CAN interface should be externally powered, or internally powered. This is important to sending and receiving correctly. The VBat jumper determines if the device is internally or externally powered.
3. Check to make sure you have at least 1 other device on the bus when using the NI-CAN card, the CAN spec requires 2 devices on the bus, or you will get NI-CAN warnings or errors.