Games : When I break demon alters I don't get mythril in terraria

I have been trying to get mythril but it comes up with all the other ores instead of mythril.im.

I'm not sure if I need to be in a specific biome for different demon alters to get mythril.oh I defeated the WoF.

I got the hammer and I don't know witch if I need a specific hammer, I defeated the WoF 2 times and it only gave me the Agile Pwnhammer and the Keen Pwnhammer.

And I also got mythril on my iPad but i just can't get it on my pc.

help would be much appreciated! :)