Why sometimes Phoebe cast two Armageddon and sometimes only one

 Title says it all.

Do you have the blood pumps boots? that increase the power of the speell and 99% times  cast 2 armaggedon, otherwise  is affected  if there are  enemies or towers around, or  leave it phoebe quiet because if you push him  to put it in a good place  is like you interrupt  the speell or something like that.