Kudu Ceramics

I decided to burnish some pieces for a pit fire I'm planning on doing soon and I've been checking on them as they dry and I've found that the burnished surface has dulled significantly with the time that it has dried! Anyone have any idea why this is happening? I thought maybe the reason is because I'm using recycled clay but I really have no idea. I've always used the same technique but it was also always on fresh clay that hasn't been recycled. The clay is white stoneware but again, same technique and same kind of clay I've burnished before with good results..