As I understand it, CANbus is designed with a loop-back, meaning anything transmitted is also read back by the device doing the sending.
The first thing I would recommend checking is the termination on your test harness. If the signal quality is sufficiently poor due to the termination on the CANbus lines, then the loop-back may not function properly. In a typical CANbus on an OBD2 port, the termination is handled inside of the vehicle, so there is no termination hardware built into Carloop.
One other thing to check, but I really doubt it is the problem, it to make sure the Carloop and the KVaser devices are not both using the same CAN ID.