Are you welding to any code?  Are you using tips or rollers?  "Tips" are usually N/A when seam welding.

Your first resource is the published literature.  If your employer cannot provide it than check the library at a local technical college - if possible, or buy some reference materials.

My first suggestion would be to consult the RWMA Handbook - available at the AWS Bookstore:  http://pubs.aws.org/p/323/rwma-resistance-welding-manual-revised-4th-ed

If you cannot find a pre-existing weld schedule, then you will have to run some experiments.

You will need to document your electrode materials, electrode geometry, force, time, and current for any experiments you run.

I'm sorry I do not have an easy answer at this point. 

Are you welding at expulsion, or below expulsion? (In other words - how much sparking is occurring?) If the electrodes are contaminated quickly then my first suspicion would be that the force is too low and/or the amperage is too high, and/or the electrode contact area is too small.  Perhaps a harder electrode material might help - but some of the harder electrode materials have concerns related to toxic metal wastes.

What are you using for a quality standard?  Are you doing destructive teardowns? 

Resistance welding is all about achieving the proper heat at the faying surface, or the interface between the two parts being welded.

As force goes up, resistance at the faying surface goes down, and heat generated at the weld goes down, and vice versa.  And, the effects of amperage adjustments are a little more obvious - increase amps to increase heat, and vice versa.