I can't drag and drop songs onto my iPod from iTunes. (Manual transfer is not working.) Please help!

Normally, I just select songs and I'm able to drag and drop them from my library onto my device. I have an iPod Touch and an iPhone 4. I want to drag and drop my songs from the library onto my iPod Touch. I have already selected manual upload instead of automatic sync. I tried suggestions from the Mac message board (since other people seem to have had a similar issue), and their solutions didn't work for me. I have restarted both iTunes, my computer, and reconnected my iPod multiple times. I don't know what's going on, and I can't find a fix anywhere in iTunes help. If anyone knows what to do, or has any suggestions, please let me know! Thanks.