Traditional taps deliver between 10 (basin) and 15 (bath) litre / minute and a bit more if you put low pressure taps on a high-pressure system! Physics tells you that if you have 24kW available you can raise the temperature of about 8.6 litre/min of water by 40°C. Uprating to 35kW combi would increase that to 12.5 litre/min. A direct measurement of the flow coming out of the taps will confirm your diagnosis of the problem.is the flow rate too much for the boiler
In my opinion, 24kW is cutting it a bit fine unless the flat has an economyl shower head fitted or occupants who are happy to wait up to 10 minutes for their bath to run.