there are several write ups about the proper way to do an introduction.
I can't tell if you are following the write up or not. some of the rules in the write up are very important to follow for a successful introduction.
I have also read from experienced owners that they have had some gliders that just won't bond to others.
A size difference can also make it harder to introduce, as the bigger ones may not except a smaller glider.
I believe getting the SCENT to mix between the pairs, more is better, pouches, fleece patches, etc. Getting a strong scent mixed around between the gliders will get them used to that scent, and make it more likely they will except each other.