Down stairs radiators not working

[moonscross]

I have added new radiators downstairs and a new boiler, everything seems to be working ok except that when CH and HW is on at the same time the radiators down stairs do not heat up, i have a y plan system with a 3 way valve which is working ok, when switched to CH only the valve moves to its posn and the radiators come on, i have tried to balance by shutting down the rads upstairs however the rads downstairs fail to ignite, i guess i have a flow problem somewhere, any suggestions

 

[MANDATE]

So it seems ok with CH only, but when HW is involved water does not want to flow to all the radiators.
I can think of two things that might be related.
Although it is not necessary to have a by-pass valve when a 3port mid position valve is used, you may well have one.
If you have added more radiators and pipe work you have increased the resistance for the water to flow. It could be the water would rather go back to boiler through by pass than go through radiators. To force water to rads close by pass a little.
On a similar tack when both HW and CH are in use, it would appear the water would rather go through the heating coil in the cylinder than the rads.
There may be a 'gate valve' fitted on the pipe work to act as a lockshield valve. Again this may have to be closed a little to force water to rads.

Apart from those I can only think maybe the pump is not as effective as it should be. perhaps silted up like mine was

 

[moonscross]

I came to the same conclusion as you except for the bypass and gate valves, however had a thought whilst the wife was watching eastenders, what if the demand for hot water was constantly on...defect on the water thermo.... after looking at, it believe it or not, i had fixed the thermo to the tank using a strap which was pressing down on the lid and stopping it from working... heyho once the demand for water was met the rads fired up.
This left the problem with flow when hW and CH was called, i have put a valve in the HW line causing a restriction this has forced flow through the rads, at the moment i am trialling the right position ...but seems to work
i have no longer the threat from the Mrs of having to sleep in the shed until fixed as all persistant DIYers have endured at one time or another.