Call Grohe customer service and tell them to send you a free replacement part and instructions/diagram: grohe:

Residential Products: We provide the following warranties on all GROHE products to the original purchaser, installed in a residential setting. This warranty is effective for all faucets sold after January 1, 1997. Mechanical Warranty: A Limited Lifetime Warranty is provided on all mechanical parts to be free from manufacturing defects in materials and workmanship under normal use for as long as the original purchaser owns their home. Finish Warranty: A Lifetime Warranty is provided on all GROHE product finishes to the original purchaser against manufacturing defects in materials and workmanship

Try removing, disassembling (take notes!), and soaking the faucet head in a mix of hot water and vinegar (50/50) for a short while (e.g. an hour or two), to help remove any tap water mineral deposits that may have made their way into the head of your faucet and that are restricting your water flow.

Faucet heads typically have very fine passages and/or aerator mesh, and it's not uncommon for a build-up of tap water mineral deposits to cause water flow problem over time. I've experienced this first hand, and a good vinegar soak did the trick.

Best of luck.