my nano keeps dropping my genius mixes whenever I sync to my pc; why won't it retain the ones it makes?

I have an ipod nano 7th gen. I like how it creates genius mixes from the songs already on it. I do not keep it automatically synched to itunes; I prefer to manually add and remove tracks as interested. Lately, whenever I plug it in to the computer, it drops all new genius mixes and keeps only old ones I made months ago. Any ideas?