Why are you using the old Series 2 CAN drivers? Is this a legacy program? Does your device support XNET? If so I highly recommend using the XNET drivers. They are superior in almost every way. That being said it is your choice and if you are more comfortable with one driver set versus another then use what you like.
Back to the question. Start with the basics. You said that you can see the CAN write works fine using another tool, can you see the CAN message come back from the ECU using this other tool? What do you see there? What I'm trying to get at is, is the problem that the ECU doesn't actually send what it should?
Assuming it is sending the data it should try opening the example in the example finder. I think it is called something like "Read write same port" where you can setup a write a frame, then read the frames back. Does this work as expected?
Maybe also look at MAX. There is a bus monitor that can look at raw frames coming in. I can't remember if there is a way to write frames there, but maybe your other tool can send the request and see if the bus monitor sees the request, and the reply from the ECU.