Please help asap! Why wasn't the Missouri compromise of 1820 enough to end tension between the north and south by the 1840s?

a. All of Texas would be a slave state, which went against the Missouri compromise.
b. Some new territories went both above and below the line of compromise.
c. Mexico had outlawed slavery, so the new land gained from them could not allow slavery.
d. The North and South continued to fight over territory by the Rio Grande.