With heavy use and age, the timing of walking foot sewing machines can become "off." This can result in such things as loose stitches, missed stitches, and even total inability to sew. In such cases, the timing may have to be adjusted for proper function. This is something that does not require a professional. It can be done by almost any sewing machine operator.
Check the timing of your sewing machine. Turn the balance wheel counterclockwise slowly with your hand. The timing is too slow if the needle is going downward into the hole below it before the presser foot is completely down. The timing is too fast if the presser foot is lifting before the needle has time to enter the hole in the foot.
Adjust the cam on the back of the sewing machine. Loosen the two screws on the cam by turning them counterclockwise with a screwdriver. Turn the cam 1/4 turn clockwise to increase the timing speed. Turn the cam 1/4 turn counterclockwise to decrease the speed.
Tighten the cam screws again by turning them clockwise with a screwdriver.
Test the timing of the sewing machine. Turn the balance wheel and observe the presser foot and needle as you did earlier. Make any needed adjustments by loosening the screws of the cam and turning the cam as needed.