I have a Toshiba Satellite 5100-503 notebook which came with 512MB RAM. Hibernation worked flawlessly for me all the time. Now I upgraded the RAM by replacing one 256MB SO-DIMM with a 512MB module. The RAM appears to work okay, but hibernation won't work most of the time. I get the "Preparing hibernate..." message, the screen goes black for a second like it always did, but instead of getting the hibernate progress bar, I am returned to the desktop (i.e. it doesn't crash or something).
I have enough free hdd space, I deactivated and later reactivated hibernation to recreate the hiberfil.sys, didn't help. I usually do not use standby mode, but I tried it a few times when hibernate wouldn't work, and standby always worked.
Any suggestions?
It appears that hibernation is more likely to fail if I have many open programms, if I have only got the empty desktop, it works flawlessly most of the time.
Any suggestion how I could get it working again?

What on earth do you need 768Mb RAM for in the first place? Try defragging the drive - the O/S may be failing to find the contiguous space it needs for the hibernate file (which will be approx. the size of your RAM of course.)