MacBook won't come out of sleep

When I close the lid to the MacBook, then open it up a few seconds later, it stubbornly won't come out of sleep. After pressing the power button a number of times, it eventually wakes up. Any ideas?

do you have all the latest firmware updates?

This actually happens to me sometimes as well.

Macbook c2d, 2 GHz, 1 GB ram, 120 GB HD.

I never pressed the power button though, I just closed it again and re-opened it.

I just did the firmware update. After a couple of successes, the problem has resumed. Sometimes opening and closing the lid a few times works but it's erratic.