If you’re not feeling it in your core it’s probably because your bum/back is too high. If you really lower your bum and keep your back as straight as possible, and hold on a little longer you should absolutely feel it.
If you find it a struggle there are variations you can try, eg rest on your hands instead of your forearms and then alternate tapping your shoulders. That one has been great for improving my shoulder strength.
I wouldn’t give up after two weeks.