Formation of gas bubbles
Leakage from the piping (such as from fittings) and the transformation of dissolved gases within liquids into gas bubbles may lead to the formation of gas bubbles inside the metering pump and the piping.
The formation of gas bubbles leads to a decrease in pumped volume and repeatability. It is recommended to degas liquids before pumping. 

Operation after leaving the metering pump inactive for a long period of time
When the metering pump is left filled with fluids for a long period of time, a pre-set pumped volume may be altered or the repeatability may decrease.
The reasons for these phenomena are as follows:
> The elastomer of the wetted parts becomes rigid.
> Anchoring of the opening of check valves can occur depending on fluids used.
> Gas bubbles have been formed within the fluid inside the pump or the piping.
It is possible that a number of no-load operations can clear these phenomena and recover the initial condition of the metering pump. It is recommended to drain all the fluids from the metering pump when it is not in operation.

Piping using soft tubes
If a soft tubing material such as silicone is used for the piping, there is a possibility that the suction pressure and the discharge pressure are absorbed by the elasticity of the soft tubing. In this case, it may lead to negative effects on pumped volume and repeatability. It is recommended to use hard tubing for the piping. 
The above list is not exhaustive; there are other factors that can influence pumped volume and repeatability.