Platform alignment issues

I have been struggling with my platform alignment ON MY STATIONARY BIKE , BUT NOT ON THE ROAD.  i have tried everything.

  • I calibrate my pedals every time before using it, so its not that.
  • I spent ages making sure my bike is standing on a flat surface, not at an angle
  • my left pedal is always 3-5 mm out, which im OK with
  • my right pedal is consistently 12-15 mm out
  • I have tried all manner of shoe inserts in my right shoe to see if that is the issue.  No real difference - 1-2 mm improvement

You help would be appreciated.