could it be that you do not have the required minimum paid contributions at FULL rate PRSI, other than the class S? See this (I.ve bolded the relevant words):
"Number of paid contributions
If you reached pension age before April 6 2002, you must have 156 qualifying paid contributions (a total of 3 years but they do not have to be consecutive). This means that you must have actually paid full-rate contributions (that is, full stamp prior to 1979 and Class A,E,F,G,H,N and S since then.)
If you reach pension age on or after 6th April 2002, you will need to have 260 paid contributions (effectively 5 years contributions but they need not be consecutive). However, if you were a voluntary contributor on or before April 6 1997, you need only have 156 paid contributions if you have a yearly average of at least 20 contributions.
If you reach pension age on or after April 6 2012, you will need to have 520 paid contributions (10 years paid contributions). In this case, not more than 260 of the 520 contributions may be voluntary contributions. However, if you were a voluntary contributor on or before April 6 1997 and you have a yearly average of 10 contributions, you may meet the requirement if you have a total of 520 contributions, but only 156 need to be compulsory paid contributions."