ubuntu 12.04LTS: sometime won't detect my iphone4s!

[ping]

folks:
not sure you guys expericence the same,  sometime my iphone4s can't be 
recognized when I connect the USB cable into ubuntu PC.
previously I had similiar issue with blackberry and in that case can be 
constantly resolved by:
	$ sudo modprobe -r usb_storage
	$ sudo modprobe usb_storage

but now this doesn't work, any known solutions?

thanks!
regards
ping