Built-in flash auto pop-up problem

This is a problem known by Canon, you will have to have it serviced.

Make sure the 420EX is set to E-TTL and set the camera to "P". If is need of more light, increase flash exposure to +1 or 1.5.

i am having the same problom try this i just found this ... http://www.fixya.com/support/t1021194-flash_not_working

Flash will automatically pop out only in AUTO mode, if the camera needs some additional lightning. In manual modes, you should use the flash button located at the left down side of the flash compartment, to manually force the flash to pop up

The flash up switch is not working properly. It probably just needs to be disassembled and cleaned. About a $125 repair at most shops.
Hope this helps.
Wes Kauffman

Can you pull it up manually, Be careful when doing this but try opening it manually, it is possible that the catch for the flash is just stuck temporarily.

most likely you have set the camera to the auto mode and you are shooting in a place with plenty of light.
your camera has a sensor which activates the flash when needed.
you can set the camera to the manual mode so that you can chose the flash to work even in the day.

DO NOT MESS with a new camera (or any other new equipment).
Take/send it back to the vendor, The purchaser has rights also. Number one it must work.
I don't know your location but in UK (& Europe) we have very strict consumer legislation.

I found the answer and fixed my flash. Look on http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00ODWf

This will describe how, if you have been using a speedlite alot, than the little tab under the shutter release side of the hot shoe can get bent down thus constantly pressing down (toward the bottom of the camera) the small button which deactivates the pop-up flash even when the speedlite is not attached. Just bend the tab up a little with a jewler's screwdriver, turn the camera off and on and voila!! Fixed!!! Dan describes pushing the tab down, but on the xti, the tab needs to be bent up to release the flash deactivation switch and fix the problem. Thanks to Dan D on photonet for saving me a service call.

Put the camera in "P" and press the flash button above the lens release. If the flash pops, than you probably just have too much light to need it when you are in auto. The flash button is not active in auto so if the xti doesn't want the flash in auto, u can't make it fire. If the flash button does not make the flash pop in "P", search this site for another post about flash not working and u will find that this could be the flash deactivation button in the hot shoe sticking. Not likely though with a new camera unless u have had a speedlite mounted alot.--

I found the answer and fixed my flash. Look on http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00ODWf

This will describe how, if you have been using a speedlite alot, than the little tab under the shutter release side of the hot shoe can get bent down thus constantly pressing down (toward the bottom of the camera) the small button which deactivates the pop-up flash even when the speedlite is not attached. Just bend the tab up a little with a jewler's screwdriver, turn the camera off and on and voila!! Fixed!!! Dan describes pushing the tab down, but on the xti, the tab needs to be bent up to release the flash deactivation switch and fix the problem. Thanks to Dan D on photonet for saving me a service call.