I deal with the same issue(s) and have found it helpful to send a short letter, notifying them of the issue as soon as possible. It can be a "soft" letter, something along the lines of "thank you for your continued support...however, we are unable to process your gift with the information currently on file...we attempt to process all gifts in a timely manner and did not want you to be concerned about your gift not being processed without an explanation..., etc." I then add a line about contacting us at their earliest convenience to update their information, or I include a form. 9 times out of 10, I will get an immediate response. It is also helpful because when they update their information on a form and send it back - it goes through our daily gift processing. It is helpful to me since I do not have to take credit card information over the phone and it provides a "paper trail." I have also found that often a donor will increase their gift amount as well.