You have a distinct advantage over other folks who don't understand how an archival institution operates. The little bit that I know has been gained by trial and error over a long period of time. If I don't find something, the answer may be --

a) they aren't where I expected them to be, or
b) what I requested wasn't what I really wanted, or
c) I made a request in terms that failed to convey my meaning, or
d) I made a request to the wrong person, or
e) requested materials are being "processed" and aren't available to patrons for the immediate future.

It's also possible that what I want has been lost or stolen from the archives, can be found at another archives, or never existed to begin with.

Any number of alternatives are possible.