[Zope] Yet another simple question

If a ZSQL Method *fails to cache* its result (internally by Zope, not by HTTP caching), when it should, what could then be the reason? I have read a few posts on this in the archives but I see no clear solutions. It seems to be that if the SQL string from Zope to the database adapter is one whitespace different it spawns a new db request. We think our SQL is the same all the time, but it fails to cache and the Z SQL Methods slow down our site. Peter