It’s probably the walls. Exterior walls typically have insulation, insulation often kills signal. There are a couple of things you can do, most of them are discussed in the FAQs on outbuildings.
My first suggestion would be to replace the garage switch with a Z wave plus switch. You can get significantly more range from zwave plus then you do from Z wave, so these days just swapping out the switch zwave plus model solves a lot of problems. And I assume aesthetics aren’t as big of an issue for the garage so you should be able to use any switch there.