A procedural oversight by environmental activists could be a legal roadblock in their effort to get federal energy regulators to reconsider three project approvals.

Legal arguments Sept. 20 over hydroelectric projects and endangered salmon in Maine focused on the activists’ oversight in trying to get the Federal Energy Regulatory Commission to reconsider its approval decisions.

A three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit pressed an attorney for the plaintiffs—led by Maine Council of...