SCADA System - Disappearing Trends

We have a SCADA system with 8 ABB AC410, 5 AC110, 2 AC70,a report station and two operator stations.

One of the AC410 is about 200m away from control room, 6 of AC41O are distributed to different process with 5 of them having additional AC110 (AVR COMPUTERS). 2 have additional AC70. One AC410 is located in control room and its the station computer also connected to a report station that displays trends from process AC410 in excel. The two operator stations are AS500 from ABB and each has an event printer also located in control room. We have five generators each with AC410 as the generator PLC and AC110 as the AVR PLC but generator 4 and 5 have additional AC70 for their auxilliaries.

The problem we are facing is that the trends on the operator station for generator 2 - 5 are broken and disappear after every 30 mins on the operator station and the trends for generator 1 (which is closer to control room ) are ok and don't disappear. The head and tailrace water levels are read from the station computer (7) and also do face the same problem.

We have MB300