This problem is by no means rare, and has featured on this forum before. I had an issue with a 1600W Ryobi, where the two-flute bit would wiggle out no matter what I did (was not bottomed out, either). The bit was good on three other routers, two of the same make but different models. New collet made no difference. The agents were able to replicate the problem, replaced the whole machine, but the problem persisted. Must have been a dud batch of spindles.
The router is lying unused, hopefully until I visit some geography where a musclechuck or equivalent is reasonably affordable (our exchange rate is putrid there was a time when it was close to 2 US$ to one Rand, now Rand 12.70 to the greenback).
+ 1 for using a slot cutter for that particular purpose.