Most "bad file" tickets I see are a first-layer problem.

Quick check before you re-download anything:

1. Bed is actually clean — IPA, then a dry wipe. Finger oil kills adhesion more than a bad model.

2. Z-offset, not flow — if the skirt is a string, go closer. If it's a smashed pancake, back off.

3. First-layer speed 20–30 mm/s — even on a Bambu. Let it weld.

4. Same filament family you calibrated — a "PLA" from a new roll can need +5°C.

If the skirt looks right and layer 3 still fails, then look at the model: unsupported islands, 0.4 mm walls, or a mesh that isn't manifold.

That's the order I debug in. File last, machine first.