Palm over screen?

In most cases, yes.  I have had CIQ widgets fail to go back to the watch face with a palm gesture.

I've never been able to get mine to do it

Try to place the hand rather flat on the watch, so that a lot of skin gets in contact with the glass. It does not work for me when I curve the hand.

Yes, this is what I have found, too. Covering the face does not do it. Skin touching a large portion of the face does work.

I was just testing this and could not get it to work. Then I tried doing it quickly. Click on any gesture and then quickly tap the face with your palm. Not a careful placing of the palm but heel of the thumb, the same kind of tap you might do with your finger to activate the screen.

Excellent. Just tested and it absolutely works. Thanks

Agreed that you need to have your palm flat on the watch, i.e. skin touching the watch including in the middle.

I have to say that the "Watch Face" default shortcut of long-pressing the back button is both natural and faster.