Watch the heater a little longer

Marlin/temperature.cpp

 
     // Check if the temperature is failing to increase
     #ifdef THERMAL_PROTECTION_HOTENDS
+
       // Is it time to check this extruder's heater?
       if (watch_heater_next_ms[e] && ms > watch_heater_next_ms[e]) {
         // Has it failed to increase enough?
           _temp_error(e, PSTR(MSG_HEATING_FAILED), PSTR(MSG_HEATING_FAILED_LCD));
         }
         else {
-          // Only check once per M104/M109
-          watch_heater_next_ms[e] = 0;
+          // Start again if the target is still far off
+          start_watching_heater(e);
         }
       }
+
     #endif // THERMAL_PROTECTION_HOTENDS
 
     #ifdef TEMP_SENSOR_1_AS_REDUNDANT