Sam,

I would ask these questions:

How many contacts do you have per board?  What is the voltage at the
rectifier when the problem occurs?  Do you have a totalizer on the
rectifier to measure the amp-hours?  If the bath is drawing current, but
the boards are only plating a thin layer, where is the copper plating (it
has to be going somewhere)?  Is there evidence of plating on the contacts?

I don't have any suggestions regarding the tape mystery, but in my
experience, the "burning" around the contact area suggests that either the
contact is high resistance (check the resistance from the contact to the
board when loading the racks) or there is very thin copper in the area of
the contact.  

Perhaps there is a thin organic film (resist?) on the copper, and the
contacts are unable to break through to form a low resistance plating
circuit.  You might try to etch a board right out of the developer before
you pattern plate.  If there is residual resist, the etching should be
inconsistent.  If the board doesn't etch, it won't plate.

If there is high resistance in the contact area, the copper will get hot,
oxidize, and the copper oxide will dissolve in the plating bath, leaving a
"burned" area.  It is autocatalytic.  As the copper is oxidized, the
resistance gets higher, generating more heat, oxidizing more copper, etc.

Hope this helps,

Dave Anderson

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