Now here's the thing; if water is drawn simultaneously from a hot outlet and certain cold outlets and/or a second hot outlet, the boiler can usually be made to fire-up and away we go. In some cases, closing off those additional outlets causes the hot water to wane.

When hot water is drawn off the Worcester Bosch Greenstar combi boiler turbine rotates and sends a voltage to the printed circuit board. If the turbine is not rotating correctly/sticking/defective it will cause this issue.
sometimes thermostatic shower mixers hot water flows section flows into the cold water section will cause a fault similar to what you are describing.

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Mohun
Repaired and serviced thousands of Worcester Boilers
Registered Gas Safe engineer