There could be several reasons.
The reader may have been having an 'off' day. It can happen through being 'energetically drained'. Issues like illness, tiredness or having done too many readings can cause the intuition to shut down.
The sitter may actually be blocking the reader from picking up their energy, whether it's intentional or not. Some sitters can be closed off for a number of reasons known only to them.
The reader may pick up on something that they feel uncomfortable disclosing and may just say they can't read the person for the benefit of the sitter. This can happen if the reader follows an ethical policy that disallows them from revealing certain aspects that could cause distress to the sitter (I've known it to happen).

Being a reader, I have times where I just can't 'tune in' and although it can be frustrating, it's better to be honest about it and admit the vibe just isn't happening.