system will not "suspend" properly; always shuts down instead

I am using a Sony Vaio laptop, model VPCEB11FM, with Ubuntu 10.10 Desktop Edition. I am a complete beginner with Ubuntu and Linux.

When the computer is supposed to be suspended/sleeping, it will not wake up properly when you press a key or the power button -- it actually goes through the whole startup process, as if it had been powered off.

I have previously used Ubuntu on this computer without that problem (it was the release that came before version 10.10).

It doesn't seem to have anything to do with the battery, and I've checked and re-checked all the options listed under Power Management.