If it's blocked in too tight the overheat sensor will shut the MW down.
Eventually that sensor will need to be changed. Usually this happens
when someone cooks for a longer period that allows the magnatron to
get real hot. Warming a coffee wouldn't do it. so if you want to use
your MW to its full potential make sure that exhaust air can move
freely. The back and bottom are generally the exhaust areas. An inch
and a half is sufficient. But of course check the manufacturer's
recommendations.