why can't i drag and drop?

I'm running leopard i think? 10.5.8. Recently when I plugged my iPod into my computer through either the keyboard or the usb on the back and i try to drag and drop the songs from my library to my ipod, it doesn't allow it. it selects the song that i click when i attempt to drag and drop. this applies to both single and multiple selected songs. any idea of what to do? it happens at random times.