We live in Canada, where the winters are very cold. As soon as the temperature drops, we can no longer get our keys into the keyholes of our exterior doors (including the one from the garage into the house). We've had someone clean the locks, so that shouldn't be the problem. Any ideas what's causing the problem or how to fix it?

heated air within the home penetrating thru locksets are creating condensation which is freezing within. get you a can of deicer or put a hair dryer in garage where you can apply a little heat to unthaw the lock. you might even cover with an old sock or glove padded with insulation may also help.

After it warms up use a little marvel mystery oil,dura lube,found at most stores do not use DW-40 it gums up compared to the others.
This should keep the locks lubed and moisture free so they shouldn't freeze up.
Squirt it right in the lock hole over kill with use put alot in it a few squirts so it is running down the door.Do this on int and ext if you dont the moisture will still work its way from the inside to the ext of locks.