I don't have, and have never had, an FL lens. That is not to say that I do have the end caps that were original to the lenses, though. I do seem to have two different varieties of Canon caps. One has the notch in only one flange, with an arrow on the back that points to the index mark. The other has identical notches in all three flanges. The Tamron cap that seems to work properly is more similar to the second variety. But maybe it is the Tamron cap that works differently than Canon intended. Should the cap rotate fully until you need to press the release button in order to remove it?