If your iPhone battery icon isn’t changing even though you’ve had it on the dock for awhile it could be because it isn’t charging or that your battery has run down close to zero and needs more time to charge than what you normally expect it to. Leave it on the dock for at least twenty minutes and then restart your iPhone by holding the Sleep/Wake and Home buttons down at the same time until you see the Apple logo appear. The icon should show as being charged at that point.