It is almost never the hardware. It is grit — and the oil you added.

A roller needs a clean, hard floor to run on — and oil destroys both

A sliding door is one of the most reliable pieces of hardware in a house, because there is almost nothing to it: a roller, a groove, and gravity. What fails is not the mechanism but the interface. Grit, sand, hair and kitchen grease collect in the groove, and the roller stops rolling and starts skidding — and a skidding roller wears a flat spot into itself in a few weeks. That flat spot is what you feel as a thump, and what makes the door hop out of the channel at speed.

The counter-intuitive part is the lubricant. Oil is sticky, and a sticky track captures every particle that passes over it. Within weeks the oil has become an abrasive paste that is worse than bare metal. The same applies to WD-40, which is a solvent and water displacer rather than a lasting lubricant: it cleans up well and then leaves a film that grabs dust. What works is a dry solid — candle wax, paraffin wax, a silicone spray that dries to a film, or graphite from a pencil. None of them trap grit, so the track stays clean and the roller keeps rolling.

Three faults, in the order you should check them

• Dirt and the wrong lubricant. The symptom is drag and noise with no tilt: the door sits square and still needs a shove. This is the most common cause by a wide margin, and it costs nothing. Clean dry first — vacuum or brush the grit out before anything wet touches it, because a wet cloth turns grit into a grinding paste and pushes it deeper into the corners. Then dry it completely, then wax.
• Roller height out of balance. The symptom is a door that hangs crooked, scrapes the floor on one side, or feels light on one side and heavy on the other. There is an adjustment screw behind a small hole at the bottom or side of the panel: clockwise raises, anticlockwise lowers. Turn it a quarter turn at a time and test — and remember that both sides must be adjusted together. Raising one side alone simply tilts the panel, because the other side pulls it back.
• The roller or the track itself. The symptom is a door that pops out at one particular point, or will not travel its full length at all. Spin each roller with a finger: a good one turns freely and silently, a worn one wobbles, grinds, or has a visible flat spot. A dented track can sometimes be eased back with a wood block and a rubber mallet, but a track deformed by more than about 3 mm, cracked or rusted through needs replacing.

The one cause nobody checks

If the door keeps going out of adjustment no matter how carefully you set it, stop adjusting and look at the screws holding the track to the header or the floor. When one or two have pulled out, the whole track can flex by a millimetre or two under the weight of the door — and that flex quietly undoes every adjustment you make. It is the single most common reason a careful repair lasts two weeks. Drive new screws into solid timber a few centimetres above or below the stripped holes and the door will hold its alignment in a way no amount of roller tuning ever achieved.