Poor first layer
Wash the build plate with dish soap and water, avoid touching the print area, confirm the correct plate in the slicer, and rerun bed levelling.
Stringing
Dry the filament first. Then verify nozzle temperature, flow, and retraction using small controlled tests.
Weak walls
Confirm the correct material profile, increase wall count for functional parts, and avoid lowering temperature until layer bonding becomes weak.
Rough corners
Reduce speed or acceleration for the affected feature and verify that the filament profile is calibrated.
Dimensional error
Measure the printed part with calipers, check shrinkage and hole compensation, and change one variable at a time.