There are a couple of questions before we
can try to give advise.

Do you have a crack in the inner layer foil itself?
   If so you should
1.  check amount of etchback - too much unsupported copper foil
   can crack.
2.  check z-axis expansion
3.  check the type of foil in the board - high temp elongation or ?

Do you have separation between the inner layer and the hole wall plating?
    If so you should
1.  determine where the separation occurs - electroless/foil, electroless/
electrolytic or within the electroless
2.  then you can start addressing the various processes which could be
responsible

Susan Mansilla
Technical Director
Robisan Lab

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