Pay Now Button missing intermittently on emailed invoices

I emailed hundreds of recurring monthly invoices yesterday for the first month of using online payments.  I sent 20 emails at a time.  The vast majority of the invoices did NOT have the pay now option appearing, but a handful did.  I've tried talking with MYOB about why this has happened but have not received a satisfactory answer.  Everything is set up correctly in MYOB.  I don't want this problem to recur in October and would like a resolution please.  Any ideas?  (It is not an option to go in to each individual invoice and untick the online option and then retick it!)