The mechanical precision needed for an electric sewing machine to sew a seam is given little thought until the machine begins dropping stitches, or worse, doesn't sew at all. It may be that the hook-needle timing is off and the machine needs to be timed. The sewing machine needle has a small notch on the back called a "scarf." Down inside the machine, under the feed dogs, is a "hook." When the needle comes down to pick up the bobbin thread, it has to synchronise exactly with the hook in order to form a stitch. There are steps you can take to adjust the timing, if that seems to be the issue with your machine.
Remove the needle plate by sliding off from left to right. Remove the bobbin found in the case beneath. Remove the bobbin case by using a flat head screwdriver to turn the screw on the left clockwise. Just turn it and lift out the bobbin case. Keep parts and screws in the order you removed them. You should now be able to see the hook and how it relates to the movement of the needle.
Set the machine to do a straight stitch, centre needle position. No zigzag. You also want to install a new 80/12 size sewing machine needle.
Rotate the hand wheel and watch the needle move to its low point. The point of the hook should meet the inside of the scarf about .5mm up on the needle. If it does not, and the hook is not picking up the thread, proceed to remove the bottom cover of the machine.
Look for the lower driveshaft, once the bottom is exposed. This is a long shaft with gears on the left side. Loosen the set screws on the right side and adjust manually by holding the hand wheel securely and turning the driveshaft. Test by turning the hand wheel until you see that the point of the hook enters just behind the needle. Then reset the set screws.
Reassemble the machine, then sew a test seam to verify that the stitches are locking in the centre of the fabric.
Some machines may have a timing belt that will need to be adjusted by loosening set screws, rather than by adjusting the driveshaft.