There are many people who found their old garage door remote doesn't work anymore after get a newer replacement, this problem is actually related to Chamerlain's garage door remote encoding system changes in past 30 years.
In 1984–2004, Chamberlain used a dip-switch encoding system, which uses 8-12 dip switches to set the code to match remote and receiver.
Then in 1993–1997, they have switched to a non-dip switch system called billion code, which has encoded the serial number in factory on chip, and the programming is by learning remote to receiver.
And then from 1997, to tackle with more and more serious security problem, they have employed rolling code technology – Security+ in their system.
With so many different coding system under one Chamerlain brand, they had to make a complicated receiver to support them all, but with one limitation, only one format is supported at a time.
And the currently supported format is decided by the first-learn remote format after full memory reset.
So let's say you have a billion code remote A and rolling code remote B, after memory reset, you have first programmed A, then you can't program B unless you do another full memory reset.
The way to solve this problem is to keep all your remotes the same coding format, either all billion code, or all rolling code.