With no input on the process or materials many assumptions will be made in this analysis. First the control and transformer are of adequate size and are functioning properly. Second the material being welded is weldable and the parts have good fit up. The force system of the welder is in good working order.
Now we can address common causes for low weld strength:
Low weld strengths can be caused by material of low strength. This will not be considered. To have high weld strengths you need a good nugget generally of modest size as prescribed by published weld schedules. Smaller nuggets produce lower strengths. Anything that retards nugget formation or growth will reduce weld strength.
Another article in this blog describes the factors that affect nugget size and growth:
WHY DO I HAVE LOW WELD STRENGTH?
Reference: RWMA - Resistance Welding Manual 4th edition