Alliance Trust Savings has received the proportionally highest number of complaints among pensions companies, according to the latest FCA data.
ATS had 21.6 complaints per 1,000 policies in force, according to the latest numbers for the second half of 2016.
A total of 10 in every 1,000 client accounts at ATS complained about investments over the period.
Old Mutual Wealth, Abbey Life and Prudential also received a high number of proportionate complaints, with 4, 2.5 and 2.3 complaints per 1,000 policies respectively.
Phoenix topped the most frequent investment complaints list for major providers, with 19.9 complaints per 1,000 client accounts, followed by Sun Life with 11.1.
Complaints about Sesame appear to be subsiding. A total of 596 were opened in the second half of last year, 95 on pensions and 231 on investments, compared to 829 in the second half of 2015.
A respite on redress
Firms facing complaints over pensions paid out £15m less in compensation in the second half of last year.
For the six months to June last year, decumulation and pension complaints resulted in £36.6m in redress, the FCA says. This fell to £21.4m for July to December.
A further £1.3m in redress was paid out for decumulation complaints.
The fall in compensation is despite the fact that, under new FCA rules, more complaints will have been reported in the second half of 2016 as previously complaints closed within one business day were not included in the data.
However, investment-related redress jumped a third from £43.5m to £58m.