Alice is completely unaware of the concept of lying, no matter how obvious.
Anyone can tell when Alice lies.
Alice just made a lucky guess - she really couldn't tell if Bob was lying.
Alternatively Bob suffers from a form of autism, Avoidant Personality Disorder or another disorder (or else he's a plain old fashioned Cloudcuckoolander) which completely messes up his body language. Alice cannot read him accurately.
Or Gregory assumes everything people say is a lie unless proven otherwise.
She just said she was guessing to keep her powers a secret.
Bob overcame this years ago through therapy, he just has a very good poker face and uses his (now redundant) disorder to explain it. Alice eventually spots his "tells".
Alice is completely taken in by Bob's obvious lies, until he tells one incredibly well-shrouded untruth. It's this that causes Alice to start doubting him.
Alice can tell whether or not anything Bob says is true, even when he doesn't know.
Zig Zagged: Alice just made a lucky guess. Oh wait, she made that up. Or did she?
Averted: Alice's powers of observation are no more or less than anyone else's.
Enforced: "We need to show that this character is hypersensitive!"
Lampshaded: "I knew you were lying because you were sweating, you refused to blink and you didn't make eye contact!"
Invoked: "I know he's lying because Charlie told me the truth, but this is a great opportunity to scare him out of ever lying to me again!"
Defied: Bob is able to explain away every argument Alice makes. He refused to blink because he had something in his eye. He didn't want to make eye contact because Alice made him nervous. He was sweating because the room was warm.
Discussed: "Wow, she's good. I hope I never marry someone like that."
Conversed: "How did you know? Are you one of those spooky Human Polygraphs they have in spy movies?"
Alice's ability to detect lies causes her to break down - she's so good at reading people that she can literally glean everything about them, including things she didn't want to know.
Alternatively, no one wants to be around Alice because they don't want a friend who's able to detect lies that well.
Alice gets over this flaw and uses her powers for good. She realizes white lies make life easier for people, and learns to tell the bad ones apart.
Alice stops telling people she knows they're lying, because people have started being very careful around her and she never finds anything out.