Canon 380ex Zoom is stuck on 55mm setting. The flash works, but the zoom flash function does not.

A good possibility is the zoom head is causing the clicking sound. As we zoom the flash, the inside of the head moves to the back and forward. Gently tap it a few times see if that helps. My guess is it needs servicing.

Check the diffuser is fully retracted. Pull it fully out then all the way back in.

Sunds like it's broken

You may need to adjust the flash exposure compensation about 1 stop or try 1 stop and if it's still not enough go a stop and a third. What's happening is the flash is being dispersed over a much wider field of view and the fall off is so much greater then at 50mm which the beam is narrower. The 380 EX works well from 35mm through 135mm. After that it still works if you are in close say 20 to 30 feet.

http://www.usa.canon.com/cusa/support/consumer/eos_slr_camera_systems/flashes/speedlite_580ex/form_display/support_by_email

Use the above form for support by email for the canon speedlite 580ex TTL Flash.

Also, there is a phone number on the website.

http://www.usa.canon.com/cusa/support/consumer/eos_slr_camera_systems/flashes/speedlite_580ex#ServiceAndSupport

Your zoom motor is probably stuck. Remove the two screws on the front side of the tiltable head. Pry off the two tilt caps using a small flat head screwdriver (they are glued on). Remove the 4 screws found under each cap. Now you can separate the two sides of the head and take out the two lenses (remember exactly how the lenses were positioned). Try pushing the flash in or out (it should move with a little force). See if the flash now responds before reassembling.

Zooming is done from the lens on a DSLR, assuming you have a zoom lens on the camera. For example, if your camera came with the 18-55mm lens, you should see the numbers 18, 24, 35 and 55 on the lens barrel with a white index line next to one of the numbers. Turn the barrel to move a different number next to the index line. 18mm is wide angle, 55mm is 'zoomed in'.

have you checked that the diffuser is pushed fully home in its slot until you hear a click - this diffuser is for close-up work only and dissables the zoom flash

compare the pin arrangement of both flashes,It is likely that this Sigma is made for some other brand.

Are you sure it's the zoom ring that's stuck, and not the focus ring? The focus ring doesn't move when the camera is in autofocus.
If it's indeed the zoom ring that's stuck, you can spend more money than the lens is worth in repairs, or buy a replacement (circa 35 dollars on ebay) for dreadfully cheap. Good luck.