Why can't I see players on the map?

I had set up a Minecraft 1.8.8 server and play together with a friend on this server. All clients and the server are Vanilla, no mods installed.

I had created Map#6 some time ago, and I copied it for my friend. It worked for some time that we both could see each other.

A few days ago, this feature broke. I can't see my friend on my map. But he can see me. The maps are identical, both Map#6.

How can I solve this?

Solution 1:

you most likely cannot see players on the map because you aren't using a locator map. By default when you create a map it will no longer show your location or player locations. If you combine the map with a compass in the crafting table it will become a locator map and give these player locations.