Many big money managers have fled startups. Venture capitalists are steering clear of high valuations and demanding that companies spend less and improve their margins—an about-face after years of profitability taking a backseat to growth. The pressure, combined with uncertainty over where the next investor check will come from, has prompted startups that seemed to be soaring just months ago to fire staff members, cut marketing spending, cancel projects and do whatever they can to make their money last.