ARTICLE IV, SECTION 2, CLAUSE 3
“No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due.”
This provision of the U.S. Constitution was widely unenforced in the North, as were the various Federal Fugitive Slave Acts and numerous affirming Federal court decisions. Many Southerners viewed this as a form of tyranny, as indicated by the language in several of the declarations of secession, and an abrogation of the famous Compromise of 1850. Of course, the long-standing animus between the North and the South also involved other issues unrelated directly to slavery, such as the protective tariffs and non-tariff restrictions on imported machinery and the “free soil” movement.
I'm still not seeing how this qualifies as "tyranny" under the commonly accepted definition but maybe that's just me.
You yourself mentioned how the federal government passed laws to ensure Article IV was enforced. That doesn't sound like tyranny to me. The Fugitive Slave Act of 1793 and the Fugitive Slave Act of 1850 were both meant to strip fleeing slaves of any legal protections they may have had. So it isn't non-compliance that the Southern states complained about, it was the fact that they thought that the fleeing slaves had habeas corpus rights and the protections of the court which guaranteed them a fair legal hearing before being extradited. Southern states couldn't handle that. Also you complain that the Northern states didn't enforce the law. But the Prigg v. Pennsylvania decision made it clear that federal laws were federal responsibility to enforce, and that states could not be compelled to do it for them. So maybe if the federal government forced the states to do their bidding, that might have qualified as tyranny. But they didn't.
Of course, the long-standing animus between the North and the South also involved other issues unrelated directly to slavery, such as the protective tariffs and non-tariff restrictions on imported machinery and the “free soil” movement.
Again, where is the tyranny of an oppressive and overreaching federal government in any of that?