Macbook wont wake after sleep

I have a recurring problem. Whenever I close the lid of my macbook and then open it, it will not wake up.
Even if it's in sleep mode for 5 hours or 5 seconds. It never used to be a problem and it would either come on right away or with a key press once opened.
Has anyone else had this problem?

Try resetting the System Management Controller. See the following Apple KB article:

Apple Portables: Resetting the System Management Controller (SMC)

Regards.

I have tried this twice now, but I still have the same problem. I'd say it happens 1 in 10 times. Anything else I should try, or are we looking at taking it back to the store?

If it's only happening 1 in 10 times, it's going to be tough to pin down. No joy from resetting the SMC probably indicates that it may be an intermittent problem. You could take it to the Apple store and have them look at it if the machine is still under warranty, but as I said above it may be tough to pin down if it's intermittent. If you decide to take it on in, make an appointment with the genius desk first.

Regards.