Are there any tricks or hints that may be helpful?
The thing about playing with a metronome is that it is largely about
attention training.
I have no doubt that, if you were to just play one single note with one finger (or just tap the top of your accordion), over and over again in time with a metronome, you could do it pretty well. Or at least get quickly to the point where you were doing it well.
What's really going on is that you are playing the note, judging where that note falls in relationship with the "click" of the metronome, then adjusting things accordingly if need be. There's a continuous feedback loop of observing and adjusting, observing and adjusting.
In fact, I feel that when you practice with a metronome,
that's the skill you're building most of all. You're not learning to play "like a robot"... you're honing the ability to play along with an
external reference. A metronome is very precise of course, but that's not the point. Even if it slowly sped up or slowed down, you should still be able to play along with it. It's the same skill used when playing with other people, which is why I always say that
if you can't play with a metronome, there's little hope of being able to play with others (at least not without annoying them).
Anyway, when you then try to play an actual tune, your brain has a lot more to pay attention to! Like a computer chip, you only have so many "processor cycles" to spare at any given moment, and you can easily use them up trying to worry about your right hand fingers, your left hand fingers, the bellows, etc. Next thing you know, there aren't any cycles being devoted to the metronome--you've completely stopped paying attention to it, and you're likely waaaay off by that point.
One solution is to play things that don't require as much attention, freeing up more of your attention for the metronome. Simple tunes. Scales and other exercises. Just the right-hand part of a tune, or just the left-hand part.
Another is to play slower. That gives you more "processor cycles per beat", giving you more time to divide your attention between playing and observing/adjusting against the metronome. (If you're playing so slow that you don't have frequent enough clicks to observe and adjust, just double the tempo of the clicks so that they're playing on, for example, eighth notes rather than quarter notes.)