Many of the Grohe faucets have a bronze screen filter on the inlet water hoses...there could be some crud in there. Have you just tried the Grohe site? They at least have the parts list, if not the install/maintenance manual. I've e-mailed them before and they've sent me a pdf of the stuff I needed. It could also just be the shutoff at the wall - try removing the hose and run it into a bucket to see if that flow is good. Sometimes the washer in the shutoff breaks up and clogs things up.