ASP.NET ajax client-side framework failed to load

Hello,

I am using WebHieratchicalDataGrid (Infragistics control) in my ASP.NET

the grid is two level. sometimes when I click on a row to expand, it works fine but sometimes it does not.

it breaks at this line

if

(typeof(Sys) === 'undefined')thrownew Error('ASP.NET Ajax client-side framework failed to load.'

);

Error: ASP.NET ajax client-side framework failed to load