Wifi is susceptible to all sorts of issues. Telus has some information on this:
I have one dead spot (even with boost) that I was able to rectify using powerline networking.
If you have no luck after trying various options in the Telus article you may wish to consider something like this:
What kind of distance is the garage away from the modem and booster? In a perfect world running a hard wire connection is the ideal way. You can add a booster closer to the garage inside your home.