ezListener
My understanding is that the allowance is €8000 pa for a married couple - allowable @ 20% (1/2 that for a single person)
This means that if you paid over €8000 in interest over the year then you are entitled to the full credit of €1600 (which is around the €134pm you quote)
At a guess what may have happened is that a combination of the declining value of the mortgage/lower interest rates may mean that this year you won't be paying €8000 in interest .. therefore don't qualify for the full relief
e.g. if the avg. balance on your Mortgage for the year is €250k and you're on a rate of 2.99% then your interest bill for the year would only be €7,475 .. thus you wouldn't qualify for the full €8000 relief