Whipping egg white

I often (not always) have issues to whip egg white into our Thermomix, it just stays liquid and eventually have to throw everything away and start over (into a Kitchenaid stand mixer where it's always successful).

I know the bowl shouldn't be fat so I every time spray the inside with vinegar, and rinse it with water before breaking the eggs. But it doesn't seem to be the solutions because, none the less, it still may fail.

Is there a (better) solution for making the whip egg white always successful ? Thanks for your time.