Map Not Showing up for Players in Mines of Phandelver

First foray with Roll20. Using the module, players can see the landing screen and the world map. When I load any of the encounter maps, it shows up for them as all black. I've looked at which layer I'm on and I'm on the Objects and Tokens layer. I've made sure they have sight as well. What am I missing?

Do they have a source of light?  Either their token needs to shed light (for darkvision), or maybe a player needs to be shedding light with the "All Players See Light" box checked due to having a light spell, torch, or whatever. Edit: These questions are assuming you are using the legacy lighting system, since I haven't really dabbled with the new experimental version yet.

So it turns out that I had to "reveal" the map. Once I did that, it all appeared. Ugh, rookie mistake. Thanks for the tip. Occam's Razor and all that. In this case the simplest solution is I'm an idiot.

Ah, Fog of War.  Another easy thing to miss.  At least you got it going!