For your description, it's either/both a problem with the payer account or Ulster Bank. Standing Orders operate broadly the same across banks, but are independent of each others. Problems with 2 seperate payer banks would just be coincidence and not attributable to a more widespread cause.
As for the payment date 'sliding', perhaps it's a case of the payment being delayed one month, and then being paid subsequently, The next payment may then be scheduled one month after the actual payment date, rather than the original schedule date.
That's just a guess on my part. I'd assume a problem with the payer account to be a more likely cause.