Why does my MBP not fully wake?
This is hard to answer outright but I don’t have the privilege of creating a comment thread and asking some questions first, so I’ll just offer some possible scenarios. 


It is just slow. This can happen if you are low on RAM. It is difficult to be patient for this though because all you have is a blank screen, and it is very frustrating.
If it is like my Macbook, and you have a password set to auto-lock or auto-lock your display after 5 minutes, and it still does not appear like it is going to wake up, then go ahead and type your password in anyway without the dialog. This will usually unlock the display and bring it out of sleep mode in one fell swoop as it finally wakes up and registers the keystrokes.


If you don’t have a password set to lock or auto-lock your screen though, I am not sure what to suggest. I have used harmless keys like the arrow keys in the past, just pressing them and alternating between left and right until it comes out of sleep mode to force my computer to wake up, but this has extremely mixed results. There have been a larger number of successes tallied doing this, but not by very much, especially if I was low on RAM before.

As to why it is like this, the possibilities range from being low on RAM or just having a slow disk, to a bug in the operating system or more aggravating, a bug in the firmware. If you can duplicate it regularly, you can try taking it into the Genius Bar and see if they can work out what the problem is there, it might even be a known issue that has already been documented and solved for some on a case by case basis, or a software update may be in the works.