722.9 transmission


since flushing the transmission at 70,000 miles on my 2012 ML 350 gas, the shift pattern is way off.  It will hard shift in the low gears on a down and up shift.  We used BG 312 fluid and changed the filter.  During this same service we changed the plugs.  The car has been back at the service garage twice and they flush again each time and reset the shift patterns but it did no solve the problem.  No transmission codes are coming up.  Now I am thinking it might have something to do with changing the plugs.  The vehicle shifted fine prior to this service.  Fluid level was check and is correct based on temp.   Any ideas????