What is the reason why the fluorine lined check valve cannot stop water?

I saw a case on a certain website where there are three pipelines on a system, but some of them check valve Sometimes it cannot stop the water, and sometimes even all check valves cannot stop the water. Even when the pipeline is disassembled, no foreign objects are found inside the check valve. However, when the valve is installed on the pipeline, the equipment will experience backflow, and the fluorine lined check valve cannot stop the water. The fluorine lined check valves are all newly purchased, and the outlet pressure is also sufficient, all above 0.4MP. The check valve is located at the outlet of the water pump. When the water pump is stopped, the pressure in the pipeline will flow back into the tank behind the pump, and the inlet pressure of the tank is originally around 0.3 MPa.

So, what is the reason why the fluorine lined check valve cannot stop water?
In fact, check valves can be said to be leak proof. As long as the leakage rate is within the specified value, it is qualified. Therefore, the above-mentioned check valves cannot stop water leakage. The following reasons may be the reasons for this:
1. The pressure difference between the front and rear of the check valve device is insufficient, which may cause leakage.
2. The valve seat may not return to its original position and may sometimes get stuck. The spring is not functioning and there is not much leakage. Or poorly sealed.
3. The installation direction is wrong. This vertical installation method, if not careful, can be installed in reverse. The valve disc can be opened by its own gravity.
4. It may be caused by a mismatch between the sealing surface and the rubber.
5. In the pipeline, if there is fine sand and gravel in the water, it will affect the reset of the fluorine lined check valve, and the contact position between the valve core and the valve body will also be poorly sealed. In the case of pressure difference, the seal will not be tight.