Push down firmly and slide? Maybe use both thumbs? I dunno, when those get stuck I just keep doing that until I finally get it to budge.

I hit it until it opens. I’m also a neanderthal.

O.K. I tested this one in the field and it worked on mine: take your hand with the index, middle and ring fingers lined up and touching one another. The thumb and pinky can be off to their respective sides, they are not necessary and place your palm, with fingers configured as outlined above, on the battery remote lid, the sliding part, with the tips of of your fingers on the arrow, if there is one. Apply firm, but not heavy pressure and pull your palm back. That should do or not. Who knows.

Or, get a bigger hammer.