PW cannot be entered on boot-up.

I am having some difficulty getting my MacBook Pro (2012, 13 inch, running OSX Yosemite with FileVault enabled) to accept my admin password on logging in, either from a warm or cold reboot. At first I believed I was typing it incorrectly; I then discovered that special characters (!@#$%^& and so on) were being omitted — that is, I would hit all the correct keys in sequence, but the input box doesn't register or display special characters. Obviously, there are no specific errors with my keyboard, as I am able to type all of these characters in the normal course of operation. "Recovering" my password from a security code or my Apple ID works without fail, but I'd rather not have to reset my password every time I boot up.