Seam welds are merely a line of spot welds. If their individual welds are weak it is caused by the same factors that cause spot welds to be weak. Some factors could be:
The size of the weld nugget
Indentation which indicates the wheel forging action will influence strength
Presence of cracks or inclusions in the weld nugget
Penetration of the nugget into both materials being welded
Brittleness of the material being welded. Tempering may be needed
The material being welded. Aluminum tends to form voids that need to be forged closed.
Proper wheel dressing
These are not all of the items to examine but they are a good start. They are based upon the proper application of Pressure Current and Time to create a good weld nugget.
Reference: RWMA – Resistance Welding Manual Section 4