I installed two new recessed LED light fixtures using 4" new construction IC housings. The problem is that both fixtures are not flush against the ceiling after installation, they are dipping down on one side 1/4" or so.
Am I doing something wrong? Is my 1990's(Minnesota, USA) ceiling drywall too thick or something? Do I need a different fixture? I'll rig something up with tape/glue/etc. if necessary but I like the fixtures so don't want to return them. How can I get these fixtures flush with the ceiling?

I've tried removing the fixtures and reinstalling with the same results. The housings are as close to the drywall as possible and secured under and to the sides of the ceiling joists. The housing itself does slide into the hole in the drywall about 1/4" I would guess but does not completely go into the room; I'm not sure if this is typical or not.
The mechanism to secure the fixtures into the housing seems somewhat unique to me. It has clips that use friction to install, but will collapse if you turn the fixture clockwise to uninstall. It is best explained via an image:

Notes
- Fixture: Commercial Electric 4 in. Recessed Soft White LED Can Disk Light Link 1 Link 2
- Box: Halo T24 New Construction 4 in. LED Recessed Housing Link 1
