Nearly six in ten complaints relating to Sipps made to the Financial Ombudsman Service between April and December last year were upheld.
The latest figures from the FOS showed the ombudsman ruled in favour of the complainant in 57% of cases during this eight month period.
Officials said they had received 634 complaints connected to Sipps – with 403 going to the ombudsman to make a final decision.
There were 968 enquiries during this period in which consumers asked for help, reassurance and explanations with problems they had encountered linked to Sipps.

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On personal pensions there were 2,288 enquiries, 888 complaints and 261 cases sent for a final ombudsman ruling. Some 27% of these cases were upheld.
For state earnings-related pensions, 352 enquiries were made. Consumers made 303 complaints, only 15 of which were handed on to the ombudsman for a final ruling. Just 2% were upheld.
Both the FOS and FSCS have forecast a rise in pension complaints for the next financial year.
The FOS is anticipating 5,400 new cases regarding retirement products in 2015/16.
The forecast is a rise of just over 1,000 compared to the 4,364 pension cases in 2013/14. By the end of 2014/15 the forecast is 5,000 cases.
Complaints and compensation claims about advice relating to Sipps will continue to rise, with an extra 500 cases predicted for the next financial year, the FSCS warned this month.
The FSCS predicted it would receive at least 5,407 Sipp claims in 2015/16, compared with 4,958 expected in 2014/15. The report stated this could be even higher.
It said: "This (the expected 5,407 Sipp claims for next year) is a provisional forecast based on current expectations, and it is possible that claims could increase above these levels."
The growing numbers of complaints connected to Sipps have been blamed for a jump in the levy advisers will have to pay next year.
The FSCS announced last week a major hike in the levy for 2015/16.
The Life and Pensions Intermediation category will pay £57m next financial year compared to £34m in 2014/15.
Mark Neale, chief executive of the FSCS, said: "It is already clear that the trend, which I highlighted last year, of growing claims arising from bad advice to switch pension savings into Sipps will continue.
"So far, we have compensated eligible investors for the pension losses incurred. "Many of these investors have also, however, suffered significant investment losses as a result of advice to invest in risky, non-standard assets such as overseas property developments."