They say they are not ready. They pull close when you step back and go quiet when you ask for consistency. From the outside, fear of commitment and simply not choosing you look identical. From inside the pattern they are different things, and the difference decides what you do next.
Short answer: commitment issues show up as someone who wants the relationship and cannot yet do the behaviours of one; the wrong fit is someone who can do them and is choosing not to, with you. The first can change slowly, with evidence. The second is unlikely to be changed by your patience.
Fear of commitment is usually an attachment pattern rather than a verdict on you: closeness feels like risk, so the person retreats exactly when the relationship becomes real. The tell is that they reach for you too. They come back, they miss you, they want it right up until they have it. Someone who is simply not choosing you does not reach; the warmth is easy because nothing is at stake for them, and the mixed signals are the arrangement rather than a conflict inside them.
Both can sit inside the same ambiguous relationship for a long time, and neither can be settled by decoding. What settles it is behaviour under pressure, over a season: does the pattern of retreat get shorter and get named, or does it simply continue? And there is a third possibility worth keeping visible: honest ambivalence that has not been said out loud, which is information rather than a diagnosis.
Ask the question that separates the two: "Do you want a relationship, and is it me you want it with?" Someone with commitment issues will usually say yes and flinch; someone who is not choosing you will usually soften the answer. Then watch the next month rather than the next evening, and let the behaviour, not the reassurance, tell you which you are looking at.
Go deeper
Why You're Still in the Situationship gives you the Relationship Readiness Assessment and the Consistency Test for telling the two apart, and the exact words for the commitment conversation. If the person who pulls away has a name for it, The Dismissive Avoidant and The Fearful Avoidant map the pattern underneath.