I have been successfully using audacity to edit my podcast for nearly 50 episodes, usually from saved QuickTime or simple “Skype Recorder” audio files.
Occasionally, when opening one of these files for editing, the display will show the file loading into the window ("0:10 remaining, etc), but the clock keeps ticking up and the file simply never loads. It’s almost like a frozen screen, but not frozen.
In the past, re-starting the program has corrected this. Now it does not.
It could be due to “read” errors. Where is the file that you are having problems with? Is it on your main hard drive, a SSD, a memory stick, a network drive…?
Or it could be due to “write” problems. When you import a file, Audacity copies the data to its “temp” folder. Look in “Audacity Menu > Preferences > Directories” to see where the temp folder is, then check how much free space there is in that location.
What format are the problem files in? And are you using the latest Audacity 2.1.2 release from http://audacityteam.org/download/mac/? At least one older version of Audacity on Mac did have a problem like you describe for QuickTime formats.