The boom in walk-in retail health clinics is showing signs of slowing. In recent months, retail health-clinic operators based in New York, Nevada, Indiana and Alabama have closed their doors, shuttering 69 clinics in 15 states. Experts say the venture capitalists and private-equity firms that backed many of the retail clinic operators failed to appreciate how complicated and expensive the clinics are to operate. Research also shows that while patients are enthusiastic about the clinics' convenience and quality of care, acceptance has been slow.