There could be a number of reasons for this.
Make sure the original remote that came with the device is in working order, by testing it on your device. If it is not responding, it might not be working correctly.
Make sure you are not performing the learning procedure under bright lighting or in bright sunlight as this can interfere.
Ensure that the two remotes are approximately 2 to 3 cm apart, and that the transmitting end of the original remote is facing the remote. Sometimes holding the key on the original remote down for slightly longer may help, and conversely sometimes quickly pressing and releasing the key on the original works better.
The batteries used should have sufficient power. Even though the remote works when using it to switch on and off devices, it might not have enough power to be able to learn additional functions. Please replace a new set of batteries
It is also possible that the memory on your remote control has reached the maximum capacity due to too many learned or other customized functions. Please remove some existing learned keys.
Some manufacturer’s remotes use a different signal, or infra-red protocol, that the One For All remote cannot reproduce. These are commonly signals that use a higher frequency. Radio frequency remotes cannot be learned from, because these signals are not infra-red based.