I don't think this is going to sound like very helpful advice, but my Xbox 360's disc tray would also not open sometimes and all it took to get it to open was to slightly lift the console (just the front side, not the whole thing) and drop it softly (but not so softly) on the surface it was on.
Nah, seriously. It sounds aggressive, but that's what I did. Press the eject button, drop the console, and bam. Tray open. And if it didn't work, try it one or two more times, and there you go.
I figure it's a Microsoft thing. PCs can also be a bit emotional like that sometimes.