Could be lots of carbon in the prechamber soaking the heat away from the plugs, or possibly the wrong plugs were installed (there are a few different length versions to fit the OM60x engine). Try letting them glow longer (10-15 seconds AFTER the light goes out) before cranking. In a cold climate, use synthetic engine oil (Mobil-1 15W-50) and this will make a HUGE difference in cranking speed compared to dino oil. I still suspect the glow circuit somehow, even if parts are new. The plugs should measure 0.6 ohms each with a meter. Just because voltage is present doesn't mean they are getting hot. If you had a shop do the work, bring it back!
