Hello,
Sorry to hear you are having trouble.
What I discovered is that only MICROSOFT applications (I tried Excel, Word, and File Explorer) have Groupy enabled on their windows. All my other apps (Affinity products, Thunderbird, etc) do not work.
Groupy will work with much more than just MS (Office) apps. I cannot test 'Affinity' products (to know if they are part of a rare group that prevents grouping), but Thunderbird does indeed group with other apps:
https://cdn.stardock.us/support/uploads/2019-06-14_15-05-52.mp4
If you have examples of others that do not (perhaps more common applications) that do not, please let me know. If there are free trials for non-common ones, please pass those along we well.
If it is a targeting issue, try this feature to see if you have more success (as a test):

Sean Drohan
Stardock Support Manager