Lenders don't like when a condo has a concentration of units owned by a single owner. It indicates a large number of rentals in the complex, more rentals and fewer primary residences mean there is less vested interest in the building overall - from an actuarial point of view, may not be a reality in your complex, but data indicates renters don't care for properties the same way resident owners do.
You need to change your marketing tactic and focus on cash buyers and investors with large down payments. Or perhaps you should consider holding onto the condo and using is as an investment/rental yourself. Are you working with a Realtor? He/she should have understood the issues up front, but certainly after the first buyer fell through. I'd consider changing Realtors in the Spring to someone who understands how it works.
You're in a tough situation. Most condo association bylaws don't allow large investor ratios exactly for this reason.
**For anyone else reading - stay involved in your condo association, when you have a majority owner who shows up to meetings and votes, the rest of the owners who don't participate in the process can get screwed by policies that only benefit that owner(s)