According to your timeline you rushed the introductions and did not give enough time for ,what I call, "scent training".
you say, "We just got our new boy a over a month ago, so we've finished the quarantine period"
then you introduced them.
from my readings, and the way we did it, we would swap sleeping pouches for a few weeks, then put cages 6 inches apart for a few weeks. So another month or more of time after quarantine passed, then after the gliders had time to smell the other and then see each other we would do the bathtub introductions.
Like you found out, many gliders love having other gliders around and are quick to make friends, but some are harder to be friendly. My wife has done about 5 introductions, with one female taking almost 2 days to warm up to the new additions, and another time one female was rejected by 3 colony gliders, so she had to be removed.
I suggest going back to scent training for awhile, then try again later, you can also see at night how they interact behind the cage bars.