If you are having trouble with your machine, your first steps is to consult user manual. Then try the tips below and consult Google and YouTube.
If this doesn’t resolve your problem, then contact me as we can try to work through it together (there is no charge). I am only currently offering consultation service (no charge) to Ottawa, Ontario area Sewists.
Note that I am a level 4 certified repair technician, however, sewing machine repair is my hobby. Therefore, I have limitations on what I can provide advice on.
Completely rethread your machine
First off, consult your owners manual/ YouTube to determine the correct way to thread your machine. Always thread your machine with your presser foot up. Make sure you follow the thread path exactly as described in your manual (don't miss a thread guide) and make sure the thread is seated securely through the tension discs in the tension mechanism. For your bobbin, pay attention to the way the thread comes off your bobbin when you place it in bobbin case (consult owners manual) and make sure you thread the thread through the tension mechanism in the bobbin case.
Check your needle
Make sure your needle matches the fabric being sewn and the thread being used. If you haven't changed the needle recently, change the needle. Even if you have changed it recently, consider changing it and/or inspect the needle for damage. Make sure the needle is installed properly (in most newer machines, the flat part goes to the back. On older machines- it varies by machine- consult user manual.) Make sure that your needle is pushed completely up in the clamp and the clamp completely secures your needle. Please unplug your machine before changing/touching the needle.
Check your thread
While using the old spool of thread you found in granny's sewing box sounds like a good idea, it usually is not. Thread deteriorates over time. Make sure you are using a high quality thread (newer) that is appropriate for the fabric and needle. Also many vintage machines do not like certain types of thread (some prefer polyester or cotton, some do not like cross wound spools). Try switching the type of thread you are using.
Check your tension settings
Kids love to turn dials. And push buttons... There's a chance that little Johnny has reset your tension for you. Not only that, your tension settings will vary depending on the fabric, thread and needle. Start mid range and see if dialing up or down the tension solves your problems (use a scrap piece of fabric). It may also be your bobbin tension- but before playing with that- you want to rule out that you cannot fix the issue by adjusting the top tension.
Check under the needle plate and bobbin case for buried treasures
Removing the needle plate is easy (promise). It's a good idea to routinely check and clean out the lint under the needle plate and by the feed dogs. Same for the bobbin areas. I like using the little mascara wands that you can buy in the dollar store as they grab lint well. Cotton swabs and pipe cleaners also work well. In particular, if you have broken a needle, make sure you find the pieces of needle before you start sewing again. Little shards of metal can destroy your machine. While you are in there, look for signs of damage- if you see any damage, you definitely do not want to keep using the machine until it is repaired. Please unplug your machine before you remove any parts.
Check your bobbin and bobbin case.
Bobbins are not one sized fits all. There are a few different types of bobbins and you must use a bobbin that is compatible for your machine. Check your user manual (or google) to determine the proper bobbin for your machine. Some machines don't do well with plastic bobbins. For example, my Singer 201 gives me nothing but grief unless I use a metal bobbin. Sometimes, cheap off brand bobbins are the problem too. I have countless trouble with bobbins I’ve purchased off of Amazon- It’s best to buy them from a local sewing store or an online sewing machine parts retailer.
Make sure when you wind your bobbin that it's winding evenly. Sometimes poorly wound bobbins are the culprit. It's also a good idea to use the same type of thread in the bobbin as you are using in your needle.
Lastly make sure our bobbin in installed properly. Make sure you have pulled the thread through the tension mechanism in your bobbin case as per your user manual. In front/side loading machines, you should hear a little click when you install the bobbin case.
Give your machine a clean and an oil as per your users manual
Following the instructions in your users manual, do a clean and oil of your machine. Pipe cleaners, cotton swabs and an old tooth brush are must have tools for cleaning. Once you’ve cleaned all of the fluff, then oil your machine as per your manual. Use only sewing machine oil- NO WD40 or other oils you may find in the garage or basement. These other oils will eventually gum up and you will be in to having someone remove the hardened oil with tooth picks and kerosene (trust me, I’ve done this many times). There are also gears that might require lube (not oil)- consult your user manual.