The latest figures showed that for the whole of 2013/14 the service has upheld 45% of complaints over cash ISAs and 43% about investment ISAs.
For this financial year so far the figures stand at 44% for both categories.
For the whole of 2013/14 there were 1,448 enquiries received about cash ISAs, resulting in 842 complaints and 1,385 enquiries received about investment ISAs, leading to 929 complaints.
There have been 725 enquiries so far this financial year for both types of ISA.
Personal pensions were near the top of the list for the most enquiries this year so far, with 1,655 (651 complaints).

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It was sixth highest, with payday loans coming out on top.
The number of personal pensions cases upheld for 2014 so far was 24%, while 20% of annuity cases were upheld compared to 32% for the whole of 2013/14.
Some 552 enquires about annuities have been fielded by the FOS this year (408 complaints), with 912 for the whole of 2013/14.
There have been just 94 enquiries about income drawdown in 2014, with a complaint uphold rate of 38%, though there were 224 reports for the whole of 2013/14 with 49% upheld.
The FOS newsletter summarised its latest statistics, stating: "We handled over 157,000 enquiries from consumers wanting our guidance and support during the second quarter of the year.
"We took on 88,038 new cases and 12,125 complaints were appealed to an ombudsman as the final stage of our complaints handling process.
"Two thirds of the new complaints we received were about PPI, and packaged bank accounts were the second most complained about product with 7,115 new cases needing more detailed work.
"The proportion of complaints we upheld in favour of the consumer ranged from 1% (for complaints about SERPs) to 88% (for complaints about card protection insurance)."