Hot water to kitchen tap not happening

[Pineapplecd]

Combi boiler fitted in 1st floor bathroom..reasonable flow of hot water to bath and washbasin. We then go downstairs to understairs loo with washbasin and hot supply ok. However we then go to kitchen tap and supply is poor. Insuffient flow through tap (if used independently) will not enable fire boiler to give hot water at kitchen tap. Pipe reduces down from 15mm pipe to 10mm pipe within body of tap. Could the problem be as simple as the tap itself or something else? i.e. component within boiler?
Anybody auxfais?

 

[ChrisR]

See if there's a piece on the end of the offending tap spout which unscrews - could be blocked.