When multiple microphone sets are used simultaneously or if there is any interference in your location, you need to change the frequency.
Make sure both the mic and receiver are on.
Find the SET button near the battery compartment of the handheld mic or the bodypack transmitter and press it quickly. 1 channel (0.2 MHz) increment per press.
If you use more than 2 mic sets simultaneously, you will need to have at least 2MHz frequency separation between every two mics.
Please refer to the video from 5:35 to 6:20.