The number of U.S. homeowners refinancing their mortgages under a revamped federal program grew in May, but critics are still pressing a federal regulator to do more to assist those with sunken property values. The Federal Housing Finance Agency said Monday that 21,605 homeowners who owe more than 105% of their home’s current value completed refinances through the government’s Home Affordable Refinance Program, or HARP, in May. That was up from 15,371 in April and only 4,168 in May 2011. However, relatively few homeowners who are deeply “underwater”–meaning they owe more on their properties than their homes are worth–have completed the refinancing process. Only about 11,000 homeowners who owe more than 125% of their home’s value have refinanced under HARP to date, with about 2,950 of those coming in May. In an attempt to tackle certain problems, the Obama administration is backing legislation by Sens. Robert Menendez, (D., N.J.) and Barbara Boxer (D., Calif.) that would making it easier for homeowners to refinance with new lenders.