My understanding is that is there are insufficient funds to meet the standing order, then no money leaves the account (unless there is an overdaft facility authorised on the account). I don't think the bank will make an attempt to transfer the money the following day as it wouldn't strictly meet the terms of the order (in this case transfer €X to Y account every Monday).
All of the above is based on my own, everyday knowledge of banks so someone may have more precise knowledge. You could also ring your own local branch (or any other branch) and ask the question in a general sense.
In terms of what your ex may be doing, there may be a number of different reasons for it, but at the end of the day, there doesn't appear to by sufficient funds in the account on a Monday morning to pay the order.