C
Centralheatking
30 kw on 9 rads should mean the place is steaming hot - I run my place with a 30kw boiler - 5 bedrooms, 3 bathrooms and hot water cylinder and never have to go past 3/4 heat - thats 16 rads and none are small. r
The OP has a boiler which modulates between 9.4kW and 30kW. He has 9 rads, which at, say, 1.5kW each is 13.5kW total load. Even if they were 2kw, which is doubtful, that would still be only 18kW. So whichever way you look at it the boiler is oversized for the heating load.
There's a good chance that the flow water temperature rises to 75, or whatever is set, but the return temperature keeps on rising, meaning the boiler then cuts out, waits and then restarts. So the mean water temperature is well below the required value, which means the rads are not giving off the correct amount of heat.
The OP should use Whole House Boiler Size Calculator to find out the heating requirement and Stelrad Elite Catalogue, first table to estimate the output of his radiators.