We get this call quite often and there is a good explanation. I assume that the wood you treated was raw wood and not painted, stained, or sealed. Powder post beetles can remain dormant for up to 10 years. Whether they are active or not depends on the moisture level of the wood. The Bora-Care will only kill the larvae of the beetles when they are active and eating the wood. When they eat the wood, they ingest the Bora-Care and die. Because the larvae may be in different stages and some larvae may be active and some may be dormant or in a different stage in the life cycle, it is perfectly normal to apply Bora-Care when you have an issue or see dust, and then have new dust re-appear a year or two later. This is because there are new dormant larvae that are becoming active in the wood again and they will die when they start consuming the wood again. So, you will see the saw dust, or frass is what it is actually called. You have to have faith in the product and trust that it is the best wood treatment product out there for a reason.
Please feel free to call the manufacturer of the product to confirm this. They may ask some further questions about your treatment, so you may want to give them a call. They are the true experts on their own products and they would be the ones to direct specific questions to. The manufacturer is NISUS, and you can reach them at 800.264.0870.