the spell arcane missiles is not working correctly as of patch 7.3.5
the spell procs you gain a stack and it starts counting down from 20 each time you gain a stack the cd refrshes and starts counting down from 20 again
when you have 3 stacks (cap) and you use one on lets say 10s the cd refreshes and you have now 2 stacks and its counting from 20 again this doesn't happen so we lose all satcks when the cd is over (even if you use one it keeps counting down from the first proc)
arcane missiles is the dps spell for arcane mages and to max the dps you should have a stack at all times with this bug we are forced to use all of our mana to get more procs because we keep losing them i hope it gets fixed soon
thank you
link to the spell and its changelog (from 7.1.5 to 7.2 it was just buffed so it should work that way in retail ): http://www.wowhead.com/spell=5143/arcane-missiles#changelog