There are different techniques for learning. Here are some to try.
1. Holding down the button on the sending remote. Most common method.
2. Tap the button once on the sending remote, don't hold it down
3. Put your hand or book between the sending remote and the 450. Hold the button down on the sending remote, wait a second and then remove your hand or the book.

You can try call URC and see if they have a solution. They have people that work on codes full time.