The Abstracta loosens up over time. Did you buy the special "hammer"
that they sell with it? I needed that to get joints undone at first.
I also carry a long steel rod that fits loosely into the tubes to
use as an internal hammer to knock small parts off the ends. In a few
years, you will have a problem keeping it together, not getting it
apart. Then, use more corner connecters, or drill and tap small
screws to permanently keep some of your parts together.
You could alter the sloped shoulders of the corner connectors with a
file or sander, if you want them to fit looser now.