No flow error on multi level Project

Hi There,

I have a multi level project 10 floors, and I'm trying to size the CWS, the CWS has 2 risers for each side of the building. Following the instructions to add floor connectors and vertical pipes I've successfully managed to connect one of the risers from ground to 10. 

However when I come to connect the other riser ( which is only 6 floors) I get an error of no flow on level 2.  I've successfully checked the systems by add in a  system marker on each floor, they all size and check as I would expect. 

I've followed previous advice given in other answers with regard to group deleting the riser and starting again, this produces the same result. I have tried connecting up the floors one at a time and checking the system and can successfully add up to and including the 2nd floor. however when I add the third floor it reports no flow on level 2 and highlights the riser area.  I can't see any issues when checking the isometric view. I'm loathed to delete the services from this floor and replace with services from another floor as this will bugger up the one part of the system which is working.

Any assistance would be very much appreciated.

Many Thanks