Edge Connector Contacts: A high-resistance contact on even a single address ($A1$–$A23$) or data ($D0$–$D15$) pin on the PROG connector will prevent the 68000 CPU from reading the game header.
P1 / P2 ROM or Trace Issues: A hairline crack along the bottom traces or an open solder joint on the P1/P2 mask ROMs.
P1
P2
NEO-273 IC: Cracked solder joints or a faulty custom surface-mount controller at the bottom-left of the PROGBK1 board.