Usually it's the sensor. Sometimes the screw just comes loose, or the connector isn't seated and they won't read, or it could be bad. It could be something else, but if it works normally when it does read, that's most likely the culprit. I've had to replace them on a Roland, Summa shouldn't be much different, they're simple to replace, just a single screw and plug the lead in. Contact this place to see if they have the one you need, it's the only US place I could find that sells sensors. This might even be the one.
Summa SummaCut Kit Sc Pinch Roller Sensor (Sn >= Xx1111-10Xxx)This is the Original Summa Part (Part No. 1NC40-67015)Compatible With:SummaCut
aaprintsupplyco.com
This is what a Roland one looks like... Not much to them