Sometimes there are disturbances that appear unexpectedly that will cause a missing weld. Even with desired stable welding currents, the growth of a good spot weld escapes. In this case shown to the left, the current is held constant (red line) at about 10k Amps for 250mS, however the dynamic resistance curve shows that the weld was never completed. Something drastic happened such as a shunting path for welding current to avoid passing through the spot weld location.

You can continue to use constant current and have a tool in the weld control identify that the weld is questionable. A tool such as SoftQ will annunciate a fault because the dynamic resistance signature is not acceptable.