Automation keeps disappearing

I will spend a lot of time creating an automation line for a track. Usually for volume. Then, I'll come back to it and the automation is gone. That is, the line is just straight. (I know the automation can be turned off and on; that's not the issue.) What could possibly be the reason that the automation disappears? I am not using the menu item (or its keyboard equivalent) to delete the automation. It seems to just disappear for no reason. Any ideas?