bosch hammer drill gbh vfr 24v?

Hi I hope you can help me with a problem i've got with my 24v bosch sds hammer drill (GBH VFR).
I took the drill to pieces after it got lots of cement dust inside it (on a job I was doing) then cleaned it thoroghly and put it back together,after i'd spent a while trying to get the case back to how it should be and secured it down with the screws again.

I popped the battery back in and tried it and all was fine apart from when I moved the selector swith to the hammer position,there was no hammer action at all and I have tried sevral times to try and repair it and this is a works drill and if I can't fix it i'll have to pay for another one so if anyone could help me i'd be very greatfull thanks everone.

Peter smith
February 2011