There are a few possible reasons for this. 

• Check your cable: Make sure your usb cable is firmly seated on both ends, and try reseating. It never hurts to swap with a new USB cable as well.
• Make sure your camera is not going into sleep mode: Unplug the camera USB and check the menu. Auto-power off should be disabled.
• Make sure you're using a Canon: PBU only supports Canon DSLR cameras. Other cameras will not function with the software.
• Be sure to REFRESH: In the Camera tab in PBU, under DSLR, click the Refresh button. If you started PBU before powering on your camera, PBU might not have noticed. 
• Restart your PC (and PBU): Try turning it off and on again. Sometimes that just does the trick.
• Make sure the Camera is set to Manual Mode: Be sure that your Canon camera is set to Manual and not to Video or another setting. It has to be in Manual in order for PBU to interface properly.
• Reset your camera to factory defaults in the menu: After this, disable auto-power off in the camera menu. 
• Make sure Canon EOS Utility is NOT running: Close EOS Utility or delete the program if not needed frequently. Only one program can control the camera at a time.