The 2 ladies that work the Family desk at Jomtien Immigration can be picky sometimes, you should go there ahead of time and ask for their checklist, I did my extension based on marriage and got a checklist from them (see attached). While you're there you should ask them specifically about the photos, because last time I came with 4x6 photos and they said I needed them to be full size on A4 paper.
Then make 2 sets of paperwork, stacked identical to each other, and sign the bottom of each paper ( I used blue ink).
Good luck.