Grohe Kitchen Faucet Membrane Replacement Instructions

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.

Found a similar design at the Delta website. The round knurled piece should unscrew. The male threaded section should also unscrew. Apparently some have a hex shape to facilitate installation. Some have an internal female hex shape that is removed by an allen wrench. This allows the retaining plate to be removed. If there is enough room between the backsplash and the spout you could try holding the piece under the sink with a pipe-wrench, vise grips or large pliers and have someone spin the faucet.