I have some photos in a folder on my PC which are copied (synced) to my iPad Air. When I change the photos on the PC and sync to iPad the photos on the iPad are not changed.
Check your Photo sync settings in iTunes on the iPad. The iPad must be plugged in and selected (or set up to sync via wi-fi and selected).
In the Photo sync settings, make sure the correct folder is selected. If you have by album/folder sync option selected make sure the folder you are editing is selected.
Hold in mind that only one level of folders will sync with the iPad. You have a main folder you select to sync. All the top level folders in that folder will become albums on the iPad. You can't have folders inside the album folders. They will be ignored.
Also make sure that the image files are standard jpg, png, or tif files. I think there are a couple more formats supported, but don't get crazy.
I find Photo Steam is an off and on thing. I suspect DropBox is cause of problem. Do a PC screen shot and it goes to DropBox and not to clip board. Expect similar with photo stream so now I just send all photos to Amazon Photo Cloud
FWIW