Quite simple really - I had 6 untapped lands - one of which was world tree. I wanted to play a 6 mana removal (descend upon the sinful) yet for what ever reason 1 of the 6 lands (nomad outpost) refused to be tapped for mana (and so I could only use 5/6 untapped lands). I checked thoroughly and could not see a reason on board as to why this was the case.