Baxi 105 HE?

I have my boiler in the loft, when i call for hot water in the bathroom sink or shower i get no hot water - but i can get hot water when called for down stairs although there seems to be no preset. An engineer says it could be lack of water pressure due to antiscaling device being blocked and pressure not being enough to activating the diaphram- why then can i get hot water downstairs - any ideas!!