Nemox - Can't Reach Temp

[Ynot]

Instead of writing a convoluted post, what's the most common reason a machine (Nemox, Lelit, Rancilio Silvia) won't reach it's thermostat's top temp? I have a 140°F thermostat in my Nemox Caffe' Del Opera and am currently only reaching 120° at best. Bad thermostat? If so, can I swap it out for one from a Lelit and possibly increase it to say 160°-165°?