Was there enough money in the account at the time WI would have tried to pull it?

Yes there was plenty

It's a month and a half after the due date, they never noticed the money never came out?  How much interest are you talking?

If you Efiled on the 15th and scheduled the payment for the same day, it may have been too late in the day for the state to initiate the debit by the time it was eventually accepted. 

I try to avoid scheduling direct debits on the same day as filing unless Im filing early in the morning.