Trouble resuming from SLEEP

Hello everyone.
When my computer had trouble resuming from sleep, it had about 27% battery life remaining, if that will make a difference...
Basically, when I close the lid of the computer it automatically sleeps. Almost always, when I open the lid, the computer automatically boots up from sleep. This time, it wasn't. Any suggestions?
Problem was solved after I plugged in the power source and the battery went up to 30%+. Is this some kind of a precaution? Thanks in advance.
-George

Edit: Problem came back again. This time, the battery is up to 72% I opened the lid, the computer did not resume from sleep. Closed it again and opened, and it worked like a charm. Software issue?
-George

Sometimes it takes about 30 seconds to wake. It helps to push enter a few times for me. Was this any help?

My iMac usually starts back up right away when I tap a button on the keyboard. Takes me usually about 2-5seconds.

Although it could be due to config differences. Has yours always been like this?
I suppose you'd also have to take powersaving modes etc. into consideration?

Yes i've been having this problem lately. Its just annoying, but it definitely never used to happen, it used to just wake up straight away when I opened the lid.

Try downloading and running Onyx or Cocktail (I prefer Cocktail)

Onyx: http://www.apple.com/downloads/macosx/system_disk_utilities/onyx.html

Cocktail:
http://www.apple.com/downloads/macosx/system_disk_utilities/cocktail.html

Thanks for the replies. I think I found out why it was behaving like this.
When the lid is closed, the light on the lower right hand corner is solid. If you try to open the lid, the computer will not resume as it is still going into sleep mode.
Now, if the lid is closed, the light is solid, and you wait until it "dances" and then you open it, it'll resume from sleep.
Hope this helps.
-George

I only ever have problems if my external hard drives are switched on when i put the MacBook to sleep, no problems waking up if only the internal drive is connected.