Quirky sewing machine

I would like to know if this is a common problem with antique sewing machines, or if i need to give it a tune up. When i am working with fabric, any fabric, if there are enough layers to angle the foot upwards / downwards even slightly, it doesnt lock in any stitches until the foot is completely level again. wondering if there is a problem with the bobbin, needle, or whatever may be the cause? i have to press down on the top of the foot manually with my finger to get it to lock in the stitches when i go over areas that would normally make the foot go at an angle.