I can think of three things.. one already covered.
Another is the ACME fitting has a cut off vavle in it. if the fill line is not properly connected this valve (like a tire valve or the valve in a quick disconnect (Throwaway bottle) is closed.
Another is the Blue Rhino thing.. Blue Rhino, in order to make sure folks who bought into their exchange system, STAYED in their system, had an extra little feature in the valve that for quite some time meant only they could refill their tanks.. I understand they have discontinued that, but...
1: I do not have a B.R. tank to my name (Thus I am not sure) and
2: You may have an older tank with this feature and
3: I am also not sure they are the only ones that stupid.