Not really strange.. its all too common actually and can leave you scratching your head. OK.. first you need to ascertain that you have no scoring switches touching when PF is down.. you should know as the game will continually score. If this is the case then a switch is adjusted too finely and you need to find it..LOL

Or.. as is a common problem with Williams EMs.. the wire loom from the PF goes to the back of the machine and up into the head. There should be a bracket at the back of the cabinet that holds this in place. Sometimes when you put the PF down this cable loom rests on the relays far back RHS and prevents the game from starting properly. Ive had this on at least 3 WMS EMs and is well worth checking out