The multimedia formats compatible by QuickTime are MOV, most MPEG files, some AVI and WAV files.
QuickTime is not working partly because your video formats are not compatible. Step 3: Wait for the process to finish, reboot your computer, and open your videos in QuickTime again. Step 2: If there is an updated available, click the Update Now button and follow the onscreen instruction to install it. Step 1: Click the Apple menu, go to System Preferences and choose Software Update. When QuickTime is not working on Mac, the first thing you should do is to update your software. Part 1: How to Fix QuickTime Not Working When Playing Video Solution 1: Update Software