If the weld equipment is not properly cooled especially the electrodes weld failure is likely to occur. Poor cooling at the electrodes can cause them to mushroom prematurely which reduces the current density at the weld face and the nugget size will reduce. Poor cooling can be caused by low flow, warm water, improperly located cooling tubes, clogged/restricted tubes and no flow.
The first check is the outbound side of the electrode water. Determine the flow rate and how warm is the out bound water. Do not burn yourself. Water should not be too hot to touch and never be less than 0.5 gal/minute flow. The desired water flow is 1.5 gal/minute. The water temperature should be at ground water temperature or good water tower temperatures. Next check the electrode holder water tube location. It should be within ¼” of the electrode inner face to prevent steam pockets.
If all of these are good then start to check the other components in the machine for heat buildup. Any copper component that gets hot will carry less current and therefore reduce the current to the spot weld. Cooling is necessary for all components of the system from the electrodes to the pads on the transformer and the transformer itself.
Make sure that there is water flow on the outbound side of all components. Sometimes lines can get plugged with debris and need to be cleaned out. Be sure that all connections are tight and clean. Frequently connections are silver plated to reduce corrosion and resistance.
If all check out good then the next step is to use a secondary current meter and force gauge. Use both to insure that each weld is being made with the proper force and current. Reference your original set up information for the job for the proper values. If the current readings do not match the original set up values take appropriate action.
Other factors that could be considered are: WHY DO MY WELDS GET SMALLER AS I WELD ACROSS THE PART?