So far the FRSA estimates about 16,000 people have come forward to make a claim.
Many of them have received lump sum payments of tens of thousands of pounds, plus monthly payments going forward.
But the union estimates at least 10,000 more are eligible but haven’t yet put in a claim.
It’s urging them to contact their former employer to find out how to put in a claim or message the union via its website, external to get help and advice.
A spokesperson for the Ministry of Housing, Communities & Local Government said: “It’s essential our firefighters get the pensions they have earned as quickly as possible.
“Fire and rescue authorities are responsible for the administration of the pensions, and the government is supporting them to address issues raised by firefighters.”
A Local Government Association spokesperson said: “Some individuals have service that goes back as far as the 1960s and Fire and Rescue Authorities have been working tirelessly in trying to trace some individuals, to ensure that they do not miss out on this opportunity.”