What can I do about postal mail not being delivered/arriving?

Specifically; the past 3 letters (in a row) to and/or from my girlfriend have not reached their destinations of me or her.
If this were a one time thing I wouldn’t be so upset, but to happen 3 times in a row like this raises suspicion and some anger—as I was to receive something very important a week ago and there’s still been no sign of it.
I’ve tried googling for things I can do about this but haven’t really found much of anything helpful. Our regular mail has been working fine, and every other letter I’ve sent out in the time period has gone through problem-free.
Advice please? :D