Some 15% of people do not know what fund choices there are when making investment decisions for their pensions or have never reviewed them, according to Avivas latest Working Lives report.
Based on 4,000 employees with defined contribution pensions, this is up from 9% in the first quarter of 2013.
The report, which examines the pensions landscape for private sector employees since the introduction of auto-enrolment, found "signs of better engagement" compared to the last three years.
The percentage of people reviewing their investment choices "several times" a year has risen from 7% in 2013 to 15% in 2016.
However, Andy Curran, Aviva's managing director, corporate and business solutions, argued more work needed to be done to encourage greater staff awareness about making the most of workplace benefits.
A quarter (26%) of respondents allow their companies to make default fund choices for them, according to the study.
Meanwhile, 23% personally examine their pension funds when they receive their annual statements.
Some 12% do so at least once a year and ensure their investments are appropriate for their circumstances.
Curran claimed staff education would prompt greater member engagement, adding: "Having brought people on board via auto-enrolment, the challenge is to educate staff further about making financial choices, especially around savings and investments, that will deliver the most positive outcomes."
A minority (5%) of workers admit they look at their options only when they move jobs and become a member of a new company scheme.
The findings also highlight that even fewer employees are seeking financial advice before reviewing, with only 4% admitting they do so.
Curran said: "We cannot be complacent and that the auto-enrolment journey does not end once employees start contributing. It is vital employees engage with the processes behind their pension.
"We must continue to make every effort to ensure people have all the support they need to make the most of their pension provision."