A system that exhibits low head pressure and high back pressure probably is suffering from:

A system exhibiting low head pressure and high back pressure is often indicative of low compressor capacity. This occurs when the compressor is unable to effectively compress the refrigerant to the required high-side pressure, resulting in a low reading on the head pressure gauge. Simultaneously, if the system is not able to draw in the necessary vapor for compression, it can lead to an accumulation of refrigerant in the liquid line or evaporator, which raises the back pressure.

In this scenario, the compressor's ability to move refrigerant through the system is compromised, impacting the overall performance and efficiency of the refrigeration system. The low head pressure suggests that the compressor is not generating sufficient pressure to meet system demands, while the high back pressure indicates an obstruction or overcharge causing refrigerant to back up.

Understanding the principles of thermodynamics related to refrigeration systems helps clarify why low compressor capacity is the root cause in this case. It prevents the refrigerant from circulating properly, leading to the observed pressures.