Filter on rad grid not getting refreshed

[Nayab]

In our implementation of rad grid, the itemsource is binded to an observable collection whose data is fetched in chunks of 1000 from the RIA services. After every 1000 record fetching, the data is appended to the observable collection. The list correctly gets updated after every fetch, however the filter doesn't get refreshed. It shows the data for the first time 1000 fetched records only. 
How do we refresh the filter ?

thanks in advance