Cannot unlock screen after prolonged period of time

[Where Where]

Greetings everybody!

I'm a new convert to Fedora. I finally got it installed (hurray!) but ran
into this issue:
If I lock the screen using mod-L and immediately enter my password to
unlock, the screen unlocks.
But if I leave my computer be for an hour and unlock, Fedora doesn't accept
my password.

Some peculiarities of my setup:
* I am using full disk encrypting using an LVM volume. Fedora setup set up
my partitions for me.
* I'm using a recent Macbook.

I have all the latest updates installed for Fedora 20. My architecture is
AMD64, and I'm using the standard (GNOME) spin.

How may I move forward?

Thanks for your attention!
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