I 'm assuming that the issue occurs when you are connected to an external monitor via TB and maybe even MD. This is a common bug that Apple has not bothered to fix yet FAIK.
1. The first time you attach it to the display after being in normal laptop mode, it always takes a little longer than you would think. If it still won't wake after about 20 seconds...
2. Reset your SMC. It'll work normally for a random period of time, then you'll have to reset the SMC again.
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