I've had a few matches recently (Sabotage) recently where I ended with a greater than 5:1 Kill/Death ratio. The Star Player challenge is unlocked, but neither match completed it.
The actual numbers were 2:0 in the first match (didn't get Flawless, I'm assuming because the game was too quick) and 1:0 in the second match (got Flawless - game was longer than the first but still pretty quick).
Is there minimum threshold of time and/or number of kills that you need to actually complete the challenge? Is it limited to specific game types? The text itself simply says 5:1 and doesn't specify any minimums or other restrictions.