They blocked Comcast from merging with Time Warner Cable. By “they” I’m referring to AT&T and Verizon (the two biggest telecom providers in the US), who were afraid of a third telecom provider establishing a national footprint and potentially challenging them across wireless and wireline. By preventing the merger through their immense political connections, they keep both Comcast and TWC as regional players who are much easier to monopolize.
So even the antitrust that goes through usually only goes through because powerful (often monopolistic) forces want to block a merger, not because it’s what’s objectively best for competition.
The original purchase of Rite Aid by Walgreens was aborted due to similar concerns, although that one ended in a revised partial acquisition anyway.
The Staples acquisition of Office Depot/Office Max was stopped as well on anti-trust grounds.
They also blocked a merger of Nasdaq and NYSE.
Those are all since 2010. I'm sure I'm forgetting a few big ones too. They should definitely be blocking more, but they have stopped some.