Text that was in the model is not on the part. A thin rib is missing. The edge of a shell has a gap in it where the wall tapers to nothing. Nothing looks broken — the feature simply is not there.
The slicer chose not to print it
This is a deliberate refusal rather than a defect, and understanding the reason points straight at the fix.
An extruder cannot lay a bead narrower than a certain fraction of the nozzle diameter, because below that width the plastic will not form a continuous line — it beads up, breaks, or is dragged away by the nozzle. A wall thinner than the narrowest printable bead therefore has nowhere to put an extrusion.
Faced with that, a slicer can either emit a line that is too wide, which distorts the model, or emit nothing. Traditionally it emitted nothing. That is why a 0.3 mm rib disappears entirely on a 0.4 mm nozzle rather than coming out slightly fat.
See it before you print it
Every slicer will show you this in preview, and it takes less time than a failed print.
Slice the model, switch to the preview, and step to a layer where the feature should exist. If there is no toolpath where the feature is, the slicer has dropped it and no amount of printer tuning will bring it back. If the toolpath is there and the plastic is not, you have a different problem entirely — look at flow and at under-extrusion.
That single check separates "the model cannot be printed as sliced" from "the machine failed to print it", which are the two halves of this complaint and share no fixes at all.
Three ways to get the feature back
Turn on variable-width extrusion. Modern slicers can vary bead width along a path, filling a wall that changes thickness with a line that changes with it. This is the single largest improvement available for thin features and it is on by default in current versions — if a model that printed on an older machine now fails, or the reverse, this setting is the usual explanation.
Lower the minimum feature size threshold. There is usually an explicit setting for the smallest detail the slicer will attempt, plus a separate one for the thinnest wall it will print. Both are safety valves against printing garbage, and both can be reduced when you know your machine copes.
Fit a smaller nozzle. A 0.25 mm nozzle prints features a 0.4 mm nozzle cannot, at a substantial cost in print time and a much lower flow ceiling. Worth it for detailed display pieces, rarely worth it for anything functional.
Or fix it in the model, which is usually better
If the part is yours, thicken the feature to at least the nozzle diameter and preferably a little more. A rib at 0.4 mm on a 0.4 mm nozzle is printable in theory and fragile in practice; at twice that it is a real feature made of two beads with a bond between them.
The same applies to embossed and engraved text, which is where most people first meet this. Text needs stroke width and depth, not just size: raised lettering under about a millimetre of stroke will be marginal on a standard nozzle regardless of the point size, and engraved text shallower than a couple of layers simply fills in.
Gap fill and its limits
Slicers also have a gap-fill mechanism for the narrow spaces left between two perimeters that nearly meet. It works, and it produces the thin scratchy lines you sometimes see in a preview between walls.
Its output is genuinely weak — a single narrow bead with poor bonding to either side — so a part relying on gap fill for structure is weaker than its cross-section suggests. If gap fill is appearing throughout a load-bearing part, adjust the wall count or the model so the geometry is a whole number of beads thick instead.
Designing around the constraint
The most durable habit is to design in multiples of your line width. A wall of one, two or three bead widths prints cleanly and predictably; a wall of two and a half produces either a gap or an overfill, and which one you get depends on the slicer version.
This is why a model that printed perfectly last year can fail after a software update. Nothing degraded — the rounding rule changed.
Materials that make it harder
Fibre-filled grades need a larger nozzle to avoid clogging, so they lose fine features first. Flexible filaments print thin walls badly because the extrusion cannot be pushed precisely enough at low flow. PLA holds fine detail better than anything else in common use, which is why it remains the default for miniatures and lettering despite everything it is bad at.