I have had this a few times now and I am by no means an expert but I have been able to Number close with this response.
If the HB says 'I don't feel comfortable yet' or 'I'm not sure' or something along those lines I usually reply with "Why not?.."
Its short and abrupt and they usually feel like they need to defend their decision.
80% of the time they will come back with 'I don't know you well enough' or some other BS.
I bounce right back with this;
"Ok. Well ask me any 3 questions, anything at all and I promise to answer 100% truthly."
This is a good point to gauge their interest and see what questions they ask.
If she asks engaging and thoughtful questions then she is interested, if she comes up with some generic crap then bin her then and there.
Once you have answered her questions, ask 3 of your own (Be creative, don't ask intimate questions).
if you manage all of that and get to this stage then ask for her number again and say something like "Now that we are like best friends... How about those digits?

"
Has worked for me multiple times.