What went wrong
You bought the transition strip before the floor was finished, and now it sits proud, rocks underfoot, or pops out of its track every time someone crosses the doorway.
The strip was treated as trim. It is not. It is a joint between two floors that move differently, and it has one job: cover the gap, follow the height difference, and let both floors keep moving.
The symptoms tell you which part went wrong. A lip you catch a toe on means the strip is proud. A hollow click means it is held on one side and rocking. A gap peeking out means the profile is too narrow. A strip cracked at its screws means somebody screwed it through a floating floor.
That last one is the expensive version. A strip screwed through the planks pins the floor at the doorway, and the pressure shows up as a peak at a seam a few rows into the room, months later, nowhere near the strip.
Why it happened
There are four profiles, and most failures come from choosing the wrong one or fitting the right one badly.
T-moulding bridges two floors that finish at the same height, with a leg dropping into the gap on each side. It is wrong the moment there is a real step.
A reducer handles a height drop. One edge sits flush with the higher floor and the face slopes down to the lower one, so the difference looks intentional.
An end cap finishes an exposed edge where the floor stops against a wall or a sliding door track. It will not span a gap between two floors.
A threshold sits in a doorway, overlapping both floors, often with a rubber or brush seal underneath. It is the right choice where the door closes over the junction.
Most failures come from buying before measuring. Too tall and it bottoms out on the subfloor; too shallow and the gap shows; too narrow and it cannot cover the expansion gap; too wide and it overhangs and rocks.
The other half is fixing. A strip should be anchored on one side or held in a track, never clamped across both floors. Screws through a floating floor take away the movement it needs.
When to stop and call someone
Stop if the height difference is too big for any strip to handle. A step of an inch or more wants a riser, a nosing or a ramp, not a piece of moulding bent over the edge. Stop if you are joining stone to timber, concrete to timber, or two floors over in-floor heating. Stop too if the floor has already peaked near the strip, because it may need relieving first, and that is a bigger conversation than trim.