"Some monster cards aren't dropping" issue isn't a glitch.

Ah, this makes senses now and I could confirm it. Both my friend and I are level 165 and attempting to hunt the future ereve cards, everything up to Advanced Knight B which is level 179 (The mage that summons ifrit and does the Flame Gear.) dropped it... after a while we decided to hunt Knight C (The pirate one) it didn't drop after over 100+ kills and wasting 500+ elixers/powers ._.... However, when we asked a fellow DrK who were 186 that was there hunting cards as well to kill for us, it dropped pretty much in 2 kills. So I'm quite sure it's following the 14 level limit system.

Also does anyone know where to find Lightning? I need the card D:, or is it in that locked room I couldn't enter?

go to the portal at the top of Knight District 2, that takes you to the other spirits.
EDIT: didn't read other comments. I guess you need to he 170 and do the empress prequest.