While holding my attack button down, I cast an ability. Afterwards, the attack never comes back. The only resolution is to release your attack, cast the ability, then go back to holding the attack down.
This becomes even more of an issue when using the shift modifier that makes your character stand still. I do the same thing as mentioned above, except nothing happens. Typically I have to wait a couple seconds, or spam my attack in order for it to work.
In previous Diablo games, you could just hold down your attack while casting abilities, and it would just go back to using your attack after the ability was casted.