Some players unable to see map layer.

I'm running a game for four players, using dynamic lighting. We are in a cave system and one or two of my players can't see the map layer. They report that they can see the shadows from their token's point of view and they can see the token. Everything else is white. So, I am guess that the map layer isn't showing up for some reason. Anyone have any ideas as to why? This doesn't show up when I click on the token and do a Control+L. It also isn't a problem on every map in my game. So, what did I do wrong?  Thanks for any advice.

Is it possible that these players aren't using a Roll20 recommended browser (Chrome or Firefox)?

I checked that, because one of these players is a notorious Explorer user. He says he is using Chrome.

Verify that the map is on the map layer and not the gm layer?

Yep, that was it. It somehow got moved off the map layer. Still, not sure why it only affected 2 of 4 players. But, the problem is solved so I am happy.

Excellent. I will close the thread.