Assuming that the software and hardware of a control system is properly working, network communications problems will usually involve cabling faults, improper cabling installation, excessive network traffic or the interface into IT network equipment. Cabling can get damaged and network connections can become loose resulting in a loss of signal across a communications span. If you're using wireless technology you can lose contact when using an unlicensed frequency and when other equipment using the same frequency is introduced into the space, causing interference.
The interface of a control system into the client's IT network is another potential source of communication issues. This involves cabling into an IT network switch, and possibly additional equipment such as a gateway that may be need to translate the control systems protocol and data format into an acceptable format for the IT network.
With field controllers where the controller uses an analog signal to communicate with the field device, such as a temperature sensor, the issue is calibration. A typical sensor may signal output via a range over a DC current (4mA to 20mA), identifying the zero and maximum level of the output of the device. These analog communication links need to be calibrated, configured, and validated to ensure the controller is getting accurate data.