Mine seems to be a bit tempermental when I try to put it to sleep too. In my case the main culprit seems to be anything using an internet/network connection, or anything the system thinks might for some reason. Like if I sign off AOL it still refuses to sleep as if it thought it were busy. If I shut down AOL completely (or sometimes the external browsers I use) it then will let itself go to sleep.
Not sure if that’s what’s going on for you, but you may try closing down applications like that that are running.
Some third-party add-ons can affect the way the system sleeps. Do you have any SCSI cards or anything like that in my machine? An outdated driver might by keeping your system awake.