Underfloor heating flow won’t get up to temp

[Rb1980]

Hi all. As above having issues with my UFH. Boiler fires up and works as normal . Issue is the return stays cold and as such when the boiler switches off when it reaches temp the flow stays about 35 degrees and the cycle continues . Have had it running with just one short loop open , pump on 3 . Doesn’t get hot . Had to turn on the radiators to get a good volume of hot water in the system and then turned on then 1 underfloor loop . Before the blending valve was real hot from the boiler but at the Manifold about 40 and return temp 35, flow meter is showing 4 ltr : min but turning the mixing valve up to max used to get return almost as hot as flow but nothing now , any ideas ? Thanks