I noticed the same kind of problem with
* mage's Arcane Missiles - channeling goes on, but no missiles are flying, moving the mage and starting the cast again helps in most cases (i'm not sure whether starting the cast again without moving doesn't help too - but in these cases the mobs are running towards me, so the position of the target is changing)
* druid's Maul - there's no miss in the combat log, it just goes off consuming rage without doing anything (i had Glyph of Maul equipped)
* DK's Heart Strike - same as the druid's Maul, consumes blood rune, but goes off without without hitting, missing or anything
In the druid's and DK's case if i don't move and just spam these abilities, they never hit, but after moving a bit there's a good chance that they start to work again.