Are you talking about the bag retainer that goes inside the plastic bag and is round and is designed to hold the bag against the wall of the drum?
If that is the one, I've read about this before.
I think any air already trapped between the bag, and the drum, will cause the vacuum inside the bag to eventually collapse the retainer.
Looking at the Oneida design, it seems they use connectors to turn their retainer into a hoop. While I'm sure that makes using it more convenient, it also means that it isn't likely a perfect fit, which means you're starting with air trapped between the bag and the drum.
I'd ditch those connectors and allow the relaxed retainer to push the bag flush to the wall of the drum. Or just make your own retainer from rolled up plastic or laminate.
Put your bag in the drum, roll-up the retainer a bit and put that in the bag and allow it to relax against the bag and push the bag tight to the drum wall. Make sure you get rid of as much trapped air as possible, then roll the bag edges over the drum.
If the retainer is close enough to the lid, you could even add a hoop on the bottom of the drum (UNDER the bag) and another hoop fitted to the top of the drum, to provide some additional support to the retainer.
Let us know if you try any of this and how you make out.
Good luck!