In my experience, clients don't really like to be called outright and asked questions like this. As the above post said, perhaps a casual text with a special offer is a more gentle approach - this gives her the chance to ignore it, or reply by text with an explanation as to why she is not making any further bookings at the moment. When people are put on the spot, it's not always easy for them to be honest if they're not happy about something! (have you ever been less than satisfied with a meal at a restaurant, but when the waiter comes round to check if everything's ok, you just nod and say it's great?)