So the common 5.5mm OD has 2 different IDs 2.5mm and 2.1mm. So if you have a 5.5mm connector and it is loose, you probably need 5.5 x 2.1. I have adapters I use in a pinch but I always put a high quality connector on the end eventually.
PS thinking about it more, lots of these connnectors are just loose and it is the female that is loose, nothing you can do about it. are you getting good electrical contact?
Jerry