Check that your Android device supports Bluetooth smart/low energy and your Android version is equal or greater than 4.4. Also check the compatibility list in the wiki.
Do not pair your Android device to the Crazyflie 2.X like other Bluetooth devices. Just connect directly through the app. If you have paired it already, unpair it and try again.
Check that the propellers are mounted on the correct motor. Check each propeller to see if it is a clock wise or a counter clock wise turning propeller and mount it according to the detailed view below.
Check that you can see a little “ring” on every propeller’s center otherwise the propellers are mounted upside down.
Check that the Crazyflie 2.X is well balanced and adjust the battery and connector until it is. Holding the Crazyflie 2.X in the small “ears” is a easy way to test it.
Vibration is a normal cause for decreased flight performance. Try balancing the propellers using this guide
Check for hair stuck in any of the propellers/motors. It can also be that one of the motors are worn out or damaged. This is a bit harder to check. It is most likely the motor towards the direction the Crazyflie 2.X takes off. Try to listen for strange sounds when blowing on the propeller attached to it.
Check each motor axis if it is bent as it causes vibration. This can be done by either applying a bit of thrust from a client or by blowing on the propeller and looking at the axis from above. If it is bent it might be possible to straighten it with a pair of pliers. If that doesn’t work the motor needs to be replaced.