There seems to be a slightly 'random' ability to produce saps. Sometimes the option appears when you go to build siege equipment on large, larger and epic walls, other times they are greyed out with a 'this equipment not needed' tool tip. This happens whether you have units with the 'Can Sap' ability or not in your siege stack. Obviously, they are always unavailable when sieging wooden walls.

I say it's random - there could be a pattern that I'm not seeing. If someone wants to have a crack at explaining it to me, I'd appreciate it.

On the other hand, it's great to see the AI using saps more frequently against me now.