Some investors in 401(k) retirement funds who are moving to grab their money are finding they can't.

Even with recent gains in stocks such as Monday's, the months of market turmoil have delivered a blow to some 401(k) participants: freezing their investments in certain plans. In some cases, individual investors can't withdraw money from certain retirement-plan options. In other cases, employers are having trouble getting rid of risky investments in 401(k) plans.

When...