Why do I only have a partial sync on my iPad?

When I connect my iPad mini to my PC iTunes recognises it & starts to update. However it only updates the Apps, but not the music or videos, before saying all is completed

Any idea why? The auto sync options are selected. I have looked through the comments here but I haven't found any comments on partial syncing

Separately I have an iPhone that syncs fine with the same library, but an older iPod touch that doesn't sync at all

Any help appreciated

Thanks