Answer:
Westward expansion caused problems for the slavery debate because with the addition of every new state, the North (anti-slavery) and the South (pro-slavery) fought over whether the new state would become a slave state or a free state. Some compromises that were made from the south was allowing popular soveirgnty which was the ability for the new states to vote for themselves whether they wanted to be free or slave. And the North agreed to the fugitive slave act which allowed people from the south to come into the North to recapture escaped slaves. Theres a video on from oversimplified on the civil war which gives a really good summary (not trying to get people to go there but it's a really good resource)