We had that issue in the beginning, front staff is the strongest point of instruction, but change your individual messages ( we have a reminder to go out a week before, and a second the day before)
Don't phrase the messages too closely: but tell the patients that they need to confirm once, or the appointment slot may be given to another patient.
That can get their attention.
As in anything new, it will take time, and often double the work(calling patients to remind/confirm appointments... and casually reminding them that you have a cutoff date for this to occur)
If your patient base is a little reluctant to utilize the internet, you will end up calling to make some reminders in any case, should you wish to keep your schedule accurate.
