Manually Moving Music to my iphone

I manually manage music on my iphone. In the past, I dragged music from my library or my purchases and dropped it into my iphone. In Sept, I purchased an album and was able to put it on my iphone, but in Nov, using the same computer, I could not. Now when I drag music and hover over my iphone I get a red ghostbuster sign. How do I fix this so that I can continue to get my purchases onto my iphone? Did something change in one of the updates?