Light fixture opening is too wide for baseplate
This is a common issue.
One solution is longer screws, if you have adjusted the screws out to their maximum length and they are still not long enough to reach through the holes in the fixture then you can get longer screws at the hardware store or try the other solution.
The other solution is to not tighten the mounting plate up against the recessed box. Loosen the screws that hold it to the box so it hangs below the box enough for the other screws to be usable.  It may take some fiddling with fixture screws to find just the right setting. It is a little tricky to get the screws and holes lined up when it is loose but it can be done.  When you tighten up the little nuts that hold the fixture on it will pull the fixture up against the ceiling so it is not hanging loose.