I have a good combi with a high hot water flow rate so enjoyed a good pressure on my mixer shower in my en suite bathroom. However now swapping my main bathroom over and put a mixer shower in there which recommended adding check valves. I have done this but since this have found that the pressure on my original shower has reduced, even when no other water is being used. I put in single check valves on both the hot and cold to the new shower and this is piped so in the theory the valves are only woorking on the new shower so cant see why they would be impacting?
