Ask yourself if you have given her all the possible support you can, after all you must have see some potential in her to allow her to pass her probation period. Have you encouraged enough clients to use her?, have you done enough role play to show her how you expect your clients to be spoken to cared for?, have you set her goal and targets in her reviews?, does she know exactly what is required of her?. She now needs a meeting with you to discuss that you are not happy with her performance and to explain everything you are going to put in place, such as various customer focused and sales goals. Give her a daily or weekly meeting to clearly lay out the sales she has to meet that day and any other objectives you have set.
Give her a time span in which she has to meet all this, such as three months, then if she achieves non of the objectives set, begin a disciplinary procedure.