What went wrong
You set the tile, stood up, and one edge sits proud of the next. That step is lippage, and by the time you see it clearly, the thinset underneath is usually already going off.
Lippage is not a grout problem. One tile edge is higher than its neighbour, so the two are not sharing a plane. Your eye catches the shadow line, your sock catches the edge, and a chair leg will find it every time.
Lippage is decided while the mortar is wet. A tile that is a fraction high when you press it down stays a fraction high forever. Grout will not lift the low tile, and pressing it later will not move it.
You can check it as you go. Run a straightedge across the joint in both directions, then crouch and look across the floor with the light behind you. Do it every few tiles, not at the end of the day.
Why it happened
Lippage almost always comes from one of six places, and only one is the tile itself.
The substrate is not flat. This is the big one. A hump under a tile lifts one corner and drops the opposite edge, and you cannot trowel your way out of it. Check with a long straightedge before anything is mixed.
The floor deflects. A timber floor that bounces lets the tile move, and tile does not bend. Stone needs a stiffer floor than ceramic, so what worked for ceramic may not work for stone.
The notch is too small. Large-format tile needs a deeper notch, because the mortar has to fill a wider, flatter space. The bag and the carton both name a notch size; use theirs.
Nobody back-buttered. Ridges alone leave voids, and the tile rests on the tops of them instead of bedding in. A flat layer on the back of the tile gives full contact.
The thinset skinned over. Mortar that has lost its sheen will not bond or let the tile settle, so the tile stays high and hollow.
The tile is warped. Large-format tile often has a slight bow from firing, and two bowed tiles set face to face always show a step at the joint. A brick-bond pattern hides a bow; a stacked pattern advertises it.
When to stop and call someone
Stop if the tile has cured and the step is more than slight, especially on stone. Grinding a cured floor is a specialist job with diamond pads and a real risk of cutting the glaze. Stop if tiles are cracking, if the floor bounces, or if lippage covers a large area. Stop too with in-floor heating or a wet room, where lifting a tile breaks the waterproofing. Some jobs are worth paying for.