Before you jump to it being their error, are you certain that your team has adequately prepared these Clients for this meeting by sending them a list of the documents you require when they confirm this meeting?
Next, find out what is missing and ask them to deliver/courier/mail it to you by a specific date so that you can complete the plan long before the Implementation Meeting. This is similar to what you want to do if they name a goal for which they don’t have enough or the specific information needed to incorporate it into the comprehensive plan. An example is a vacation home: they don’t know how much a home costs in the area they want to buy. They need to contact realtors and discover how much they’ll need to achieve that goal. So while you’re completing the plan, they’re doing homework on their goal.