Before anyone mentions this, yes, I have bug reported this.
After a very frustrating experience with a Fall of Truth run last night where I experienced many failed castings of spells with Quicken Spell toggled on (mostly haste and healing spells), I asked my guildies if they have been experiencing issues with Quicken Spell. The answer was almost unanimous: yes, and the problem has been around for quite some time. Up until the last year or so, I have played primarily clerics where often my role in raids would be to cast offensive and heal spells while standing still in a relatively safe spot; Quicken Spell was merely a safeguard and I did not experience this bug very often. However, recently I have spent a lot of time playing a bard arcane archer, which, while a fun build, is heavily dependent upon Quicken Spell to allow me to move quickly and strafe while casting. I have discovered that while attacking or moving (or strafing AND moving), Quickened Spells will be quite commonly interrupted somehow. I've found the only way to guarantee that a Quickened spell is cast is to stand completely still and NOT attack. This is somewhat counterproductive for obvious reasons.
So I want to appeal to the devs to fix this bug and put priority on it. I also want to appeal to players to bombard the bug report tool with reports of Quicken Spell not working and the conditions under which they have noticed it not working. A lot of characters depend on Quicken Spell combined with tactical movement to avoid being hit or to kite. Maybe if enough people sign this thread, discuss it, and bug report it, the devs will pay attention.
Report your bugs with Quicken Spell here!
And if there is some kind of work-around or tips on how to use Quicken Spell effectively, please share them here!
Edit 1: For further details, I did a little focused testing today. I did not have auto-attack on (and as a general rule I do not use auto-attack except for very specific circumstances, such as attacking Lolth), I was simply holding down the attack button. Sometimes the casting animation will run, and when it does, the visuals for the spell effect occur (such as when casting haste, the yellowish circle will appear centered on my hard target or myself if I have no hard target). In such a case, the SP will be spent. However, the buff does not appear on my buff bar or any allies nearby, and it appears for all other intents and purposes that the spell did not go off successfully.
Other times, the casting animation will not run, nor will the visual effects appear, but the cooldown will start. In this event, the SP is not spent, and spamming the button will get the spell cast properly.
So there are two aspects to this. The first appears to occur only with Quickened spells and DOES use up SP without applying the buff; the cooldown also begins and no amount of spamming the button will get it to go off. The second appears to be the more generalized problem with interrupting any action while performing another action (such as attacking and casting a spell at the same time, attacking and drinking a potion at the same time, or attacking and activating a boost at the same time). This second I am aware of, and I know the workaround is to just spam the second action until it goes off. But as yet, I have not figured a workaround for the Quickened spell issue aside from standing still and not attacking.