There are a few things you can do about this. Some of these are conventional wisdom, some I’ve been learning lately through experimentation (with other finishes, not Proofgrade).
Sand the surface, or lightly engrave it, where the glue is to be applied, to remove the top layer of varnish and provide a textured surface for the glue.
Add some score marks that the glue can get its fingers into.
Try different glues that are better able to stick to varnish, e.g. poly glue, epoxy, craft glue.
CA glue seems to bind strongly but is said to be brittle. A good trick is to use a few dots of CA glue around the edges, for a quick set to obviate clamping while your wood glue dries.
Use a mechanical fastener or joint. It can be invisible, like a spline or dowel joint, which is easy to do with a Glowforge.