First of all you need to wait a little after making a trim change to let the aircraft settle down to the new trim setting. It's even more important in the sim because the aircraft don't seem to be as stable as you find in real life. In a real small airplane you should settle down with only a couple bounces when you make a change in pitch. In the sim it can be five or six oscillations.

Other than that the trim isn't perfect in MSFS in general. It seems you are always a little undertrimmed or overtrimmed. In a real aircraft you have almost infinite adjustment. In the sim it's in steps and they always seem to be a be either too much or too little. Of course that doesn't really matter because in a real aircraft you will almost never fly in conditions where you can just trim it and let go of the controls and the airplane flies straight and level. Same thing in the sim except for a different reason. If it's too bad however you can play around with the "elevator_trim_effectiveness" in the aircraft.cfg file. Just be sure you make a backup copy.