It depends on where you have your camera set to save the images. If it is set to save it to the phone itself, that is probably why. You need to go into the camera app and change its settings to save to the SD card instead. Then you need to set your gallery to either look for images from the SD cards only or to look on both the device and the card for them.
What it says is available isn't always what is. Especially if you have programs that constantly add data to the phone. Messenger, facebook, etc. Also, before doing anything try restarting the phone (not reset, just restart, as in turn it off and back on). Sometimes that is all you need to do.