Difficult to say. Obviously, if you have the stitch length set high enough (at least 2.5 or higher) so the feed dogs should pull the fabric through the machine, then something is definitely off. If it were a purely mechanical machine, sometimes old oil has dried and "glues" the mechanical movement together so the parts seize. This happens in the older mechanical machines when they are not treated to regular cleaning and oiling. If this is the case, your machine could benefit from a professional cleaning and oiling. However, if a part is worn or broken, parts replacement could be required.
The other thing to consider is what kind of fabric you are sewing. If it is light-weight fabric like chiffon, sometimes the presser foot pressure adjustment needs to be increased. This applies more pressure between the presser foot and feed dogs so the feed dogs can grip the fabric better. Try stitching on different weights of fabric and see if there is any difference. Most machines do have a pressure adjustment. You just need to know where to look for it on your particular model. It's usually placed at the top left side of the machine and could be a "plunger" type, a screw that is tightened or loosened, or on newer machines, a dial. On older machines, sometimes it is under the top cover at the top of the presser foot bar. What Pressure Adjustment on Sewing Machine
Aaaak! Is this a sewing machine or serger? This diagram may help:
or the manual: