CFMU vs callsign

[civil-twilight]

I'm not sure if this is the right place for this post, but maybe someone can help me.

Why is it that I can't see all callsigns in the cfmu? When I try on certain callsign the cfmu database says that the aircraft operator does not excist, even though I am certain that it is being used by an operator and I may also be able to find the specific flight if I know at what airport they are staying at. But why does it give an error when I am searching on the callsign (three letters)?

[Mister Geezer]

It is something that Eurocontrol can only answer. Just because an operator does 'not exist' does not mean there are no active flight plans linked to that operator in question.

[10W]

Are you trying to access from an operational piece of equipment or through the public portal ?

If there is an operational need to know the data, then I am sure Eurocontrol can assist.