One of the common problems that cause dropouts is antenna placement. The antennas for the wireless system are buried in the back of or under a rack. Then the metal will reflect the wireless signal. Or the receiver is placed in a closet or under a table, where it is not in line of sight with transmitter. It will compromise the signal that receiver can pick up. Furthermore, being so low to the ground, the signal has to travel through all of the people in order to get to the receiver while the human body will have more chances to absorb, interfere or absorb it. It also results in compromised reception. But do not think the higher you place the receiver the better the signal you will get. Because some venues would have metal dropped ceilings. So the best practice is to keep receiver at the place that is visible from where the microphone is being used.
Besides the placement, it is not a wise idea to randomly set or change frequencies. Wireless mics can interfere with each other. If using multiple FIFINE wireless microphone, you had better to keep the frequency channel at least 3MHz from each other.