Housing charity Shelter is urging local authorities to do all they can to help tens of thousands of people struggling with the charge and other welfare reforms.
The Discretionary Housing Payment fund has to be allocated by the end of the month, but Shelter said Government figures showed more than £15 million remained unspent by the end of 2013. A quarter of authorities had spent less than 30% of the DHP budget.
The charity wants each local authority to do more to promote the fund to people affected by the bedroom tax, officially known as the spare room supplement, and other benefit changes. It is also calling for earlier unsuccessful applications to the fund to be reassessed.
Graeme Brown, Shelter Scotland director, said: "Shelter Scotland has been campaigning for an end to the bedroom tax since before it was introduced and we managed to get the Scottish Government to make £20m extra available to local authorities to help people struggling to pay their rent.
"We are concerned that there is still a significant number of struggling tenants eligible to receive this money who are not getting it - whatever the reason. It would be shameful if all the money now available isn't spent.
"Failure to do so just doesn't make sense and would mean that, instead of getting help, some people would face the very real threat of mounting rent arrears, eviction and homelessness."
A spokesman for local authorities' umbrella group Cosla said: "Most additional funding only became available to local authorities in October this year. Local authorities across Scotland are doing everything that they can to ensure that those in housing need are helped through the award of a discretionary housing payment."