Most cutting problems come down to the feed/temperature/tension balance. Here is a quick diagnosis table and the fixes.
The wire bows backward
The wire lags behind its endpoints — the cut is inaccurate and the wire may snap. Fixes: slow the feedrate, raise the wire temperature, increase wire tension, or use thicker/nichrome wire. Bowing is the #1 symptom of feeding faster than the wire can melt.
The kerf is too wide / edges are rounded
The wire is removing too much foam. Fixes: lower the temperature, raise the feedrate, or use a thinner wire. This is the opposite problem to bowing — the wire is too hot or too slow.
Corners are rounded or gouged
The wire bows differently through a turn. Fixes: slow down into corners, add a tiny dwell at sharp points, increase tension, or design gentler corners/radii. See Cut quality.
The wire keeps snapping
Fixes: reduce feedrate (less mechanical load), check tension is sprung (not over-tight when cold), reduce temperature if it is glowing bright, and use nichrome rather than stainless for hot, long spans.
One side of a morph scorches
The two wire ends trace very different perimeters, so one moves faster and melts a wide groove. Fixes: reshape one profile so the perimeters are within ~50%, or raise the feedrate so even the faster side stays under the melt threshold. cncfoam.com warns when the mismatch exceeds ~1.5×.
Parts come out the wrong size
Fixes: calibrate steps/mm on each axis (see Motors & drivers); account for kerf (parts cut slightly undersize, holes oversize — see kerf compensation); and confirm your material-block/origin setup matches the real machine.
The cut twists when it should be straight
On a dual-tower machine, the two towers are not perfectly matched. Fixes: calibrate X/Y and U/V steps/mm independently, check both towers are square and the rails parallel, and confirm the profile is loaded with the correct left/right alignment.
Rough or marked surface
Fixes: keep the feed steady (speed changes leave lines), lead in from an edge rather than starting on a show face, clean baked-on residue off the wire, and prefer XPS over coarse EPS for the smoothest finish.