Could be any of these:

Broken lead on the switch capacitor.

Broken lead on the switch resistor.

Coil has a broken wire making intermittent connection.

Cold solder joints on the connector that operates the coil on the cpu board.

Cold solder joints on the driver transistor.

The transistor for the lower jet bumper is Q81 for Sorcerer.

You'll definitely want to do as Ken suggests, but I have a game specific observation that might prove relevant. If the angle of your playfield is too shallow, that particular pop will often fail to register contact.

When I first got my Sorcerer, I noticed that the lower pop would often not fire when hit. I followed the usual troubleshooting best practices, but could find nothing wrong. A while later, while fixing a busted seam in the cabinet, I figured that I would replace the rusty legs and leg levelers with new ones while I had them off to clamp and glue the seam. I set the front levelers all of the way down and the back all of the way up. This made, at most, a one degree difference in the angle of the playfield. The change was like night and day. The lower pop fires correctly for just about everything but the most slow, floaty contacts now.