As mentioned, pushing the paperclip does require a bit of force and works in the majority of cases.
However I have seen CD/DVDs that were stuck with an edge up not allowing the CD tray to open fully. In that case you simply remove the DC from the computer and tap it gently a few times, on a hard surface and it can dislodge the CD then use the paperclip to open the tray.
In an extreme case if you need the stuck media you may have to open the drive, which is not that difficult, by popping some screws out then removing the case shell top, then you can have access to the media and get a better visual on what is going on.