Below are some common problems/answers.

This is probably due to a bad radio-connection (using same channel as WiFi?).

Most of the time this is either due to the fact that there's dirt (like hairs) stuck around the axis of the motor between the motor house and the propeller or that the propeller is too close to the motor so there's friction between the two. If neither helps check that the propeller is attached in the right way.

If there are other 2.4GHz devices (WiFi, Bluetooth, etc.) around there might be running on the same channel so try switching it in the bootloader configuration block. You can also try adjusting the radio bandwidth. If you set it to 250kbit/s you will get good range outside but inside the reception/range might become worse. Since the rate is lower the data spends longer time in the air which increases the probability of interference. Changing the speed to 1Mbit/s or 2Mbit/s will give better results under these circumstances.

There's spare parts available at Seeedstudio

Try to flash the latest firmware using the instructions here and firmware here.