This is the most common cause by far. If the thread tails aren't held or secured at the start, they get pulled straight down through the needle plate on the first stitch and the needle unthreads itself almost immediately.

• Before you begin sewing, hold both the top and bobbin thread tails towards the back of the machine
• Keep hold of them for the first few stitches until the thread is locked into the fabric
• Note: if your machine has a top loading bobbin with an easy or quickset feature, you may only need to hold the top thread tail — the bobbin thread is managed automatically. Check your manual to confirm what your machine requires

If only a short length of thread is pulled through the needle eye after threading, it will pull straight back out as soon as you start sewing.

• After threading the needle, pull at least 10-15cm of thread through
• This gives enough of a tail to hold and enough slack for the first few stitches to form correctly

If the take-up lever is in a mid or low position when you begin sewing, it pulls the thread back upward on the very first stitch — and the needle unthreads.

• Before starting a seam, turn the handwheel towards you until the take-up lever is at its highest point
• Make this a habit at the start of every seam — it only takes a second and prevents this problem entirely

If the thread isn't correctly seated in the tension discs it won't have enough resistance and will slip back through the needle eye when you start sewing.

• Raise the presser foot before re-threading — this opens the tension discs so the thread seats properly
• Follow your machine's threading path carefully all the way from the spool to the needle

Most machines thread from front to back or left to right. Threading in the wrong direction means the thread pulls free almost immediately.

• Check your manual if you're unsure of the correct threading direction for your machine
• If you've been using an automatic needle threader, try threading manually to double check

The wrong needle or one that's incorrectly fitted can also cause unthreading.

• Make sure the needle is the correct type for your thread and fabric — a needle with too large an eye for the thread can allow it to slip out easily
• Check the needle is fully inserted with the flat side to the back and tightened securely
• Replace the needle if it's been in the machine a while — a worn needle can have a slightly damaged eye that doesn't hold the thread properly

Once you've worked through the steps, test on scrap fabric — hold the thread tails as you start and check the needle stays threaded through several lines of stitching before returning to your project.