This can happen when the sealer or primer has not dried adequately. Even though these products are designed to be easily sanded, if they haven't hardened sufficiently, particles can stick to the sandpaper. 

To prevent this problem, it is crucial to adhere to the minimum drying time recommended by the manufacturer before sanding the product. Trying to sand a product that hasn't fully dried can lead to using more sandpaper and not obtaining an optimal sanding outcome.