In your system, the PM is effectively the chief executive. I know, I know, technically the GG is chief executive, or very technically, the representative of the chief executive, but for practical, lawmaking and governing purposes, the PM is the chief executive. Because the PM is always from the majority party or coalition, the PM's legislative agenda will pass Parliament, except in cases of a major rebellion in the party or coalition.
It's not that way here. All laws have to be proposed in the House of Representatives or the Senate (though they can be presented for consideration by the President). Then the bill must be voted by a majority of the originating chamber (in some cases a supermajority of 60% in the Senate). The bill then goes to the other chamber, where it is debated and voted, but the other chamber can pass a different version than the version passed in the originating chamber. In that case, the bill then goes to a "conference committee," the differences are reconciled, and then both chambers must vote to pass the reconciled bill.
Then the bill (now an Act of Congress) goes to the President, who may sign it, in which case it becomes law, or veto it, in which case it does not become law unless both chambers of Congress vote to override the veto by 2/3 majorities (almost impossible).
Unlike Canada and Britain, the two chambers and the Presidency need not be of the same party, and in fact, it is rare for one party to hold both chambers of the Congress and the White House. Representatives, Senators, and the President are separately elected, by more-or-less popular vote. The Democrats had all three for only two years of the Obama administration, and the Republicans had all three for only two years of the Trump administration. Thus, it is far easier to "kill" a bill in our system than in yours. This is further exacerbated by the fact that our Senate, unlike your Senate or the Brits' House of Lords, is a fully equal partner in the process, and not the mostly toothless collection of old farts y'all have. For any one of the three to oppose a bill is almost always sufficient to kill it.
This, rather than corruption, lies, or whatever else you may imagine, is why when a Presidential candidate says she will do this or do that, it often doesn't happen. It's not because he was lying or corrupt, it's because he is unable to get his agenda through the Congress.