How was the fugitive slave law of 1850 strengthen?

It was a stronger version of the old Fugitive Slave Law which had fallen into disuse.

Congress was having to make a big gesture of appeasement to the South, to compensate for the increasing difficulty of creating new slave-states.

The new Act backfired badly. The Northern public resented being treated like unpaid slave-catchers, and 'Uncle Tom's Cabin' was written as a protest against it.