The proposed merger between Spirit Airlines and Frontier just can’t get off the runway. Spirit announced late Thursday it would again postpone a vote on the deal, a sign that it lacks shareholder support for the merger in the face of a rival bid by JetBlue Airways. Spirit delayed the vote by a week, until July 15. That’s the third time Spirit has put off a vote on the Frontier deal. JetBlue’s offer is worth more in current value, about $3.6 billion in cash. But Frontier’s cash-and-stock bid would let Spirit shareholders keep 48.5% of the combined airline.