Why is pump suction pressure low?
Improper inlet conditions are one of the most common reason pumps fail to produce their expect pressure. Inlet issues may also cause ongoing reliability problems. One culprit is air entrainment or lack of prime. It is important to make sure that system is designed so that liquid floods the pump to prime it.
The compression ratio will be high from the low suction pressure, causing low volumetric efficiencies. The compressor will simply not pump much refrigerant, and all components in the system will be starved of refrigerant.
Overcharging leads to overfeeding the evaporator with refrigerant, which leads to higher suction pressure. Another possibility is having oversized EXV causes an overfeed to the evaporate, which increases your suction pressure.
Not enough air flow OR too much air flow can cause problems with the system. When there is not enough air flow through the evaporator or a restriction in the air flow, the Suction pressure is below normal because the refrigerant flowing through the evaporator picks up less heat than normal resulting in lower pressures.
Greater Current Draw. The greater rate of gas molecule generation in the evaporator of an overcharged refrigeration system would mean that the specific volume of the refrigerant in the suction line will be low.
Refrigerant overcharging can also result in liquid refrigerant running too far into the evaporator coil, leading to a too-low suction side pressure and a reduction in cooling capacity.
Suction pressure should be kept at less than 200 mmHg in adults. It should be set at 80 mmHg to 120 mmHg in neonates.  The catheter size used for suction should be less than 50% of the internal diameter of the endotracheal tube. A common conversion is that a 1 mm diameter is equal to a 3 French.
Entrained air or gas/vapor can have a detrimental effect on the suction performance of a pump. Air or gas will expand in the inlet of the impeller, which blocks the liquid flow through the impeller, much like the blockage from cavitation vapor bubbles, both of which cause a decay in the pump discharge pressure.
Adding refrigerant decreases leaving evaporator superheat by increasing system pressure and increasing the flow of refrigerant through the evaporator. The suction line saturation temperature will go up and the spread between suction saturation temperature and suction line temperature will decrease.
Check your hose and accessories for a clog. If no clogs are found, check the hose for a hole or crack - even a small crack can cause a loss of suction. If a crack or hole is found, replace the hose. If you do not feel suction at the inlet, turn the vacuum off and unplug from the outlet.