Tripels usually take some time to carb up. This is due to the higher gravity and higher ABV. Yeast have more resistance to deal with in the higher alcohol environment and just takes longer to happen. I usually don't bother sampling them until 6-8 weeks. Even then, they are best after bottle conditioning for a few months. You will find it amazing at 6 months. Just let them sit.
"Look, let me explain something to you. I'm not Mr. Lebowski. You're Mr. Lebowski. I'm the Dude. So that's what you call me. That or His Dudeness... Duder... or El Duderino, if, you know, you're not into the whole brevity thing." -The Dude