CAN receive & RTX timeout issues

I am using an LPC2378 (MCB2300 demo board) in a CAN communication setup to a single node. The other node is using proven code (on a LPC2129), I am just trying to talk to it using my own code.
I'm using the CAN library built into the RTX, which seems pretty simple.

I perform a successful send:

message.id = CANid_getinfo|1;   //CAN board id#1
message.data[0] = 0x01;         //get info from sensor #1
CAN_send(1,&message,0);

This is followed by:

while(1){
   manifest = CAN_receive(1,&message,0xffff); //0xfff = wait indefinitely for msg
      if(manifest == CAN_OK){
         //do stuff here

The message is received properly and the other board begins transmitting.
The problem is I never get out of my CAN_receive function - the system then hangs forever in the os_idle_demon task (note the calling task for the above code IS still running).

OK, maybe the CAN bus has an issue - but if I change the timeout (RTX is using 10msec ticks)

manifest = CAN_receive(1,&message,0x10)

it STILL freezes. The RTX timeout won't work for any value!
Only by changing the timeout to zero does the code continue - but I never get CAN_OK, only CAN_TIMEOUT_ERROR.

I am convinced this is something stupid I'm missing, not being too familiar with using the RTX kernel...

Thanks for any help