MODBUS connection, interruption in Data stream

We are currently working on a substation automation system, and transfer our measured values via MODBUS RTU protocol over the RS485 connections. Sometimes (and it is frequently happening) a special thread of numbers (energy values) are not transferred to the master system. And this happens while the communication is OK (all the other signals from the same device are received at the same time) and the mentioned energy signals are transferred to the system some other times.

Does anyone have any idea about the cause of this?

Thanks for all the comments in advance.
Shiva