1. The dialog seems too early or late
My first take on this problem would be to check if the audio track has been accidentally nudged out of sync. Slip the audio track back and forth on the timeline and see if the animation falls back into sync.
Sometimes a shot may look mysteriously out of sync even though a frame by frame inspection shows the mouth shapes have been accurately timed to the dialog. Try offsetting the mouth poses a frame or two earlier than the corresponding sound - this can improve the readability of lip sync. The exception is the closed mouth sounds B, M and P, which should fall right on the closed mouth pose.
2. The mouth shapes are too big
If the character's mouth moves through big shapes for open mouth sounds like A, E and O - this can look pretty grotesque. An animator tackling lip sync for the first time might have referenced a mouth shape chart like Preston Blair's famous 'DIALOGUE' page (Google it!) and created key mouth poses that are over sized. Lip sync mouth shape charts often show exaggerated poses - to make the point, rather than to be followed verbatim. Scale down the size of the extremes of the mouth shape and try it again.
3. Too many mouth shapes
The mouth is moving through so many shapes in the dialog that it looks jittery and unnatural. It's a common mistake to try to hit every phoneme with a key mouth pose. I like animators who have a mirror on their desk - the best way to check what's really going on with speech is to rehearse it in a mirror. Identify the important shapes that define the word, and skip over the lesser ones. Generally you should try to create lip sync with as few mouths shapes as you need to make it convincing - real speech is like that - the mouth doesn't move all that much. An extreme example is anime - a limited animation style, typically achieving lip sync with two mouth shapes - open and shut! Millions of anime fans never complained about this.
4. The wrong mouth shapes
If the mouth is closed when it should be open, wide when it should be narrow, or not closed on a B, M or P, it should be pretty clear that the wrong mouth pose is being used. I know it sounds obvious, but I've seen it many times and unless it was somehow accidental - there's really no excuse for it. As a supervising director once told me - animators who do this should be publicly flogged. That might be a little harsh - but you get my drift.
5. No follow through
It's not a good look when a mouth slams shut - back to the default closed position at the end of a word. Unless the word ends on a closed mouth shape, the last shape of the spoken word should hold for a while.
Finally, there are no hard and fast rules for lip sync. What works for one character design or animation style, won't necessarily work for another. It may seem bewildering at first, but once you nail it - it's magic. And lip sync is just a subset of facial animation - coming soon.
Sherazade The Untold Stories (2017) is an animated TV series initially developed by Hahn Film. Clip is from episode 117, directed by Steve Bristow.
Got any more lips sync tips and tricks? Post 'em as a comment.