Ours slips over the brackets occasionally, too. I now start the bed lift then step over and gently assist the lift to keep the mattress engaged with the brackets. Ours has always stayed put once fully tilted.
Another problem we had was that the bed lift button on the multiplex panel didn't actually lift the bed when we first got our Aria 3401. Power cycling the Firefly system resolved that.
Editing to add that tucking covers under the foot of the mattress makes the problem worse, so we just tuck the sheets then make sure that the mattress is fully engaged with the brackets.
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