If you have the Sentry 12-key controller, it is possible that your kiln uses a type-k thermocouple, but your controller is programmed for the type-s. This will make the kiln underfire. You can change the thermocouple type in the controller under Options.
Did the firing time suddenly or gradually become longer?
Firing time can lengthen suddenly due to a drop in voltage, a bad relay, or a broken element. Does firing time vary depending on time of day or season? That would indicate voltage drop.
Has anyone recently worked on your kiln? Some kilns with series-parallel elements will continue to fire even if they are mis-wired. But they will not fire as hot as they should.
If firing time gradually became longer, it is usually caused by worn elements.
You could check the amperage using an ammeter. By checking the amperage of each individual relay inside the switch box, you can quickly locate a burned element or relay. (An ammeter check should be done only by a qualified repair person, since you will be exposed to live wires.)