Dont want to put you into a situation where you may void your warranty... However the "occasional" chemical backflush makes an amazing difference in terms of the reduction of bitterness.
Could be that your machine is dirty. Soak the P/F baskets and showerscreen in some espresso machine destainer for 30 mins. and rinse "thoroughly" if you cant get some dedicated espresso machine destainer try a bit of nappysan and some hot water.
Backflush at your own peril. That said I do it myself every 4-6 weeks (generally when bitterness starts to become a little prevalent). Plenty of good websites to explain how to do this.
Remember though backflushing can void your warranty. OTOH your 3-way valve may eventually fail if it becomes encrusted with rancid old coffee oils too.