Trouble with getting PhD recommendations - almost 60% turnover in faculty
The most common and most valuable letters come from the professor(s) that were your academic advisor(s), or that otherwise directed or participated in your thesis work.

Not to be acerbic, but you made the mistake of treating your master's program as a place where you went to sit and look at powerpoint slides, and made use of your professors as if they were just there to read from powerpoint slides.  A typical response to a recommendation query from such a professor is likely to be "I don't think I'm a good person to write you a recommendation, because I will not be able to speak to your strengths and abilities."  

They say this to you because there is a well-established unwritten and unspoken protocol in PhD admissions, that of the "damning with faint praise" letter.  The committee reads that letter as "this professor either had nothing to say about the candidate, or chose not to say bad things about the candidate.  In either case, the professor had nothing GOOD to say about the candidate."

Beyond that...It is quite acceptable to ask an inactive professor (retired, on leave, etc.) to pen a recommendation.  It's entirely their prerogative whether to accommodate you though. Only your direct thesis advisor feels a burden of responsibility to write anything at all.