Quantum PLC drop module error problem

WE have a quantum PLC with CPS 111 power supply module, CPU 434 module and CRP 931 remote master module. Three drop modules has been configured in the system. In HMI there is provision for drop failure indication. Recently one drop module indicated error in the HMI and readings connected to AI modules etc went to zero or erratic reading was showing. So suspecting that drop module would have failed, we changed the drop module.
After which it was observed that occasionally drop error in HMI came, but corresponding AI values in HMI was updating . How to solve the issue completely. Thanks in advance