Answer:
The Missouri Compromise was a sensible and clear arrangement that simply fixed a line of latitude, North of which slavery would be illegal. It kept the peace for thirty years.
The admission of California to the USA, following the Mexican war, rendered the Compromise unworkable, and a new deal had to be worked out, to allow the new state to be declared free soil.
It was this new Compromise (1850) that raised tensions everywhere, largely because of the Fugitive Slave Act, which allowed for the employment of official slave-catchers to hunt down runaways. Then came 'Uncle Tom's Cabin' - and all the rest of it.