How do I fix radiators that come on at different times?
I think you need to balance the system;  as water will always take the easier option.   

So when the system first comes on most of the water will go to the radiator with the shortest pipe run, once the thermostat rad valves on that radiator shut down, then the water will find the next easier option.

So by turning down the “lockshield“ values (the value with a cover over it) on the radiator(s) that gets hot first and turning then up on the radiator that take longer to get hot, you can force the water to go to all of  your radiators.