This is the most common cause of the machine not cutting at all. It's very easy to accidentally disengage the knife, particularly if you've previously been doing a flatlock or another technique that requires it to be switched off.

• Check the knife engagement lever or switch is in the correct position for cutting
• Refer to your manual if you're unsure where this is on your specific model

Overlockers have two blades - an upper and a lower. Understanding which one is likely to be the problem saves time.

• If the cut edge looks ragged, torn or uneven rather than clean, the blade is likely blunt and needs replacing
• The upper blade does most of the cutting work and is the one that needs replacing most often - start here
• The lower blade is more durable but can be damaged if you've sewn over a pin - if replacing the upper blade doesn't fix the problem, inspect the lower blade for nicks or damage
• General wear on the lower blade over time can also affect cutting quality even without a specific incident - if you sew regularly and haven't changed it in a long time, it may be due

Blade replacement is straightforward but needs to be done carefully - the knife is extremely sharp.

• Turn the machine off completely before attempting to replace the blade
• Follow your manual carefully - upper and lower blade replacement procedures differ and vary between models
• Never touch the blade edge with your fingers
• Dispose of the old blade safely - wrap it in card or tape before putting it in the bin

If the machine is cutting but trimming too much or too little fabric from the edge, the cutting width may need adjusting.

• Locate the cutting width dial or setting on your machine - check your manual if unsure
• Adjust and test on scrap fabric until the trim width looks right for your project

Very thick fabrics or too many layers at once can be too much for the blade to cut through cleanly, even when everything else is set up correctly.

• Try reducing the number of layers if possible
• Slow right down when approaching thick seams to give the blade time to cut through cleanly
• If you regularly overlock heavy fabrics, check your machine's specifications to confirm the maximum fabric weight it's designed to handle

Before returning to your project, test on scrap fabric of a similar weight. Check the cut edge is clean and even along the full length of the seam before continuing with your actual work.