Now here is the rub: it is not a frequency issue. The frequencies were properly coordinated to avoid interference between the units. So what is the root cause? An analogy will explain.
You are sitting in a dark room and your eyes have adjusted to the darkness. In the far corner, there is a nightlight with a very low wattage light bulb. You can easily see the glow emitted from the nightlight. Suddenly, a bright ceiling light is switched on and then off. For some time, you can no longer see the nightlight glow because your eyes have been "de-sensitized" by the bright light.
The dim nightlight is the SLX transmitter. Your eyes are the SLX receiver. The bright ceiling light is the PSM transmitter. Because the PSM transmit antenna is very close to the SLX receive antennas, the SLX receiver is overloaded (or "swamped") by the strong PSM signal. The SLX receiver can no longer "see" the tiny signal coming in from the SLX transmitter. The SLX receiver has been de-sensitized by the strong PSM signal. Just like your eyes were de-sensitized by the bright ceiling light.
How much stronger is the PSM signal because of its proximate position to the SLX receiver? Let's assume that the SLX transmitter and the PSM transmitter are emitting the same signal strength. However, the SLX transmit antenna is 20 feet away from the SLX receiving antennas, while the PSM transmit antenna is 0.5 feet away. This difference in distance provides a difference in signal level of nearly 30 dB! The SLX receiver is being "blinded" by the strong signal from the neighboring PSM transmitter.
The solution is to separate the PSM transmit antenna from the SLX receive antennas. But how great must the separation be? There is no hard and fast rule. The answer is "farther is better." As a starting point, ten feet of separation is suggested.
Though it may be convenient to mount a PSM transmitter in the same rack a wireless mic receiver, it is not best practice. The transmit antenna and the receive antennas will be too close for comfort.
Arcana - mysterious or specialized knowledge
The terms "antenna" and "aerial" are often used interchangeably. Originally, a rigid metallic structure was called an "antenna", while a flexible wire form was called an "aerial". So a radio station emitted a signal from its "antenna" which was picked up by the "aerial" on the exterior of an automobile.