How can a trim system be inaccurate?
When it is virtually impossible to trim off the pressure as required.and turning the wheel results in either over or under trimming, despite one's careful efforts.
By way of comparison, the C152 and C172 both trim accurately, as did your PA38 ny the sound of it.
BTW, please note (CFI too) that I am not knocking the whole PA38 fleet, but commenting from experience, the 4 I flew were similar in this respect and not up to the job.