Trouble finding studs in garage
If the drywall was hung on top of plywood with nails or screws there's a decent chance those won't align with the studs, which is why your magnetic studfinder could lead you astray.

The plywood has to be anchored to studs, and those should be 16" on-center.  So you only have to find one as a reference point.  As an alternative to opening enough of the wall to probe for a stud you can use the fact that the plywood anchors are magnetic and roughly 16" on center, combined with the existing holes that failed to find a stud, to map out likely stud locations.