I have used the radgrid in one of my application just like the related grid where I'm passing the Id from first grid to populate the second, so i think that it is not the problem with the Radgrid.
Would you please check whether your radgrid is placed inside any control so that while rendering to html it's clientId will be different from RadGrid1
for example if the grid is inside rock itz clientid will be like this "RadDock1$C$RadGrid1"
so you have to use the ControlParameter
statement as shown below.
Here is the online demo of related grid
showing how to pass the selected Id from the Radgrid as controlPaermeter.