(Nexus 7 with TL-WN722N card)
If your card appears to go to sleep when your tablet/phone does, simply click "Take WIFI lock" and "Take wake lock" in the terminal emulator. This is not a permanent setting, so you have to repeat it each time you re-open the terminal.
I think this should be put in the official tutorial if others have this problem too. Could save some headache for nabs like me.