Permanent synch issues

I have a sansa 4gb fuze. I would go to the library, and synch it to their computers, but I would never permanent synch it. I bought a laptop recently, and I permanent sycnched it on my windows media player for my laptop. While it was doing it… it did not synch the whole way through. I didn’t take out the cord. The computer said it was not able to synch it all the way. I didn’t think it meant much, and continued putting songs on my mp3 like usual. Since then it doesn’t always save the music I put on it after it gets done synching, and removing the cord. How do I fix this issue?