I had just got done cleaning the spokes on my Trek 7.6, '09 vintage when I noticed the computer, a Bontrager node 1.1 was not working. Not being much of a computer guy and with the LBS near, I thought I would let them work their magic.
Well, they said my batteries in my fork sensor needed replaced. O.K. I got that taken care of. No pairing occurring! Maybe I had the batteries in wrong? Checked and rechecked. Still nothing. Called the Trek gurus themselves and confirmed batteries go in positive side up. Got it. Still a no go. Last but not least, the magnet. Cleaned it off with glass cleaner and turned it around and boom! Success! Just a little heads up if you are struggling with pairing your unit.

Cleaning the magnet didn't do anything.

I'm guessing it was too far from the sensor.

If you have the Bontrager sensor (the speed/cadence one, at least), note that it has LEDs that indicate that it's getting a signal from the magnet. It only flashes about ten times.

I don't think it goes into pairing mode unless it gets a signal from the magnet.

Im sure your right. Im very new to this game. I tried moving the magnet (I thought) every position possible. The last thing I did was to clean it. It didn't look dirty tho. So your idea is probably right. Touchy & sensitive this stuff is. I know it sure got the best of me for a day or so.

I have about 8000 miles on a Node 2.1. It seems rather reliable to me.

I have had similar issues and they even foxed the local (very good) mechanic (Ashley at Push Active, Devizes, England) until he read the small print on the sensor battery lid. Apparently this says that prior to fitting a battery, you should put it in the wrong way/upside down for a few seconds before fitting it the correct way. I guess it draws out a charge. I haven't seen the instruction myself, I am too busy enjoying the resurrected contraption to open it up and risk losing it again but I trust Ashley.

I know this post is a decade old, but for anyone swapping out batteries in an old, non-Bluetooth bomtrager duotrap, putting the battery in backward before putting it in correctly actually worked with nothing else did. Thanks for the tip.