While poor prep will lead to poor adheasion, I think the most common reason for paint flaking is the paint has become too thick. You get 6-8 layers of paint, any paint, and it is likely to start flaking. The magic of an ablative paint is if properly used it will not produce a buildup resulting in paint flaking. The trick with ablative paint is to not put any on unless it is needed - i.e. you do not recoat your boat every year.
If the only problem you have with your bottom is flaking, I would not disturb or change the barrier coat that is already in place - it is apparently is doing its job, why mess it up or do anything new?
You need to remove the buildup of the old paint, you don't need to remove all traces. Roll on one or two coats of a light color ablative, then two coats of whatever color ablative bottom you want. In future seasons, only add paint when and where you can see the light color lower coat.
Do it right and seasonal prepping your bottom for launching can only take an hour or two a year.
Contact Interlux at 800-468-7589 yachtpaint.com - the official site of International and Interlux paints
to resolve questions about compatibility of their paints with what you already have, see http://www.yachtpaint.com/Images/15_20650.pdf
. They are very helpful on the phone.