Quote Originally Posted by Seasoned Alcoholic
Another irritating feature with sieges is the 'randomness' of saps. Sometimes they are available to build as siege equipment, other times they are greyed-out and not possible (this is against all 3 types of stone walls btw). I've noticed that they may become available to construct if you wait a turn or so (IE maintain siege).
I agree, it's very annoying. When it happens, just save the game and reload. The option to build sapping point will be available.