This is a very late reply but putting it here in case someone is searching for info. We saw a lot of reports on quick synth failing at the start of Dawntrail when above the recommended craftsmanship value. We tested this a lot and came to the conclusion that most of the time this bug happens if your tools (main or offhand or both) are lower item level. Even when your recommended craftsmanship is well above the value. And it only happens on Dawntrail recipes (so 91+). Have reported it in game as bug report but it might still be in game. Had someone do a test and they failed 3 out of 5 items quick synthed, just by equipping an item level 540 offhand instead of an item level 620 scrip offhand.

To clarify: this is for sure a bug. Quick synth is not supposed to fail when above the recommended craftmsanship. Have only seen this happen on Dawntrail recipes.