Perhaps it happens because enters in DFU mode directly.
Or perhaps you have jailbreaked your iphone.
Do this steps:
1.- Do a backup of your personal data and software if it can be done.
2.- Turn of the phone
3.- Plug the usb apple cable to your computer.
3.- Keep pressed the () central button and plug the usb cable to the iphone.
It should enter in DFU MODE showing the itunes logo and the USB PLUG with an arrow.
Start Itunes and perform an iOS UPDATE.
that its not the right way to go to DFU mode, if you want to put your device in DFU mode just follow this steps. plug your iphone on the pc or mac then press and hold the power and home button for ten seconds, then release the power button while holding the home button and wait the confirmation for the itunes your phone is in recovery mode. and you will recognize your screen goes black. no itunes logo nor cable logo. if you can see the itunes and cable logo, your on a recovery mode. its not DFU mode.