The District’s annual delinquent property tax auction failed to generate bids on more than half the lots up for sale, leaving $24 million in real estate taxes still unpaid. But the Office of Tax and Revenue isn’t giving up on its collection efforts. Quite the opposite — it’s ramping up. OTR sold the liens on roughly 1,265 properties during the three-day September sale out of more than 2,700 lots auctioned. The sale generated $11.2 million, well short of the $35.9 million owed to the city in taxes, interest and penalties. The properties that did not sell are “bid back” to the District. Starting soon, officials say, OTR will launch a “special deed sale,” where it puts unsold, still-delinquent properties on the block to anyone with the cash to pay what’s owed.