Fannie Mae and Freddie Mac last month saw sharp percent declines in new issuance of single-family mortgage-backed securities, and they are capturing less of the conventional-conforming market.The two GSEs generated $65.94 billion in single-family MBS in August, down 20.8 percent from July, according to a new Inside The GSEs analysis. Monthly volume has seesawed through most of 2015, driven in part by how much seasoned-loan business they get. Most of the August decline in new issuance stemmed from a sharp drop in Freddie’s vintage-loan activity. After securitizing $8.00 billion of loans aged more than three months in July – most of them delivered by Bank of America – Freddie securitized just $99.6 million of such loans in August.