Check those lines. Either a leak or a kink. Do you have the lines the right distance down in the tank? That two stage pump might be the answer. (If yours system does not have one. Your pump filter might be stopped up. Has your oil filter been replaced. How is your nozzle? You might have to call an oil burner man. The oil we are getting now is not very clean. Have to change the oil filters a lot more offen than we used to. Sludge is a real problem now. In your tank, lines, in your pump filter and line filter.