Hello there,
Is the zone you refer to i.e. kitchen dining room, a particularly large area, or long run of pipe? You may need to balance the system, in that you need to ensure that longer circuits receive more heated water by adjusting the valves at the manifold, plumber who installed should really do this. Have a similar problem at the moment myself with my upstairs UFH, but I think that is also what I need to do. However, the wee tool that I need to adjust the valves was lost during the cleaning up of the house (only in 6 months), bugger. Was looking at it wondering what it was, just lying there on the ground for months, and when I realised what it was, and why it was needed,it had up and disappeared.
Could also be a blockage in that circuit though, hopefully not.