I'm running WIndows8 (all updates have been applied to both) on both my laptop and desktop computers. I placed the same folder of pictures on both computers and the File Explorer shows all 415 folders in that pictures folder. On the laptop, the Photo app shows all 415 folder. However on my desktop it only shows 207 folders.

Please try to re-add all folders into Pictures library.

Meanwhile, you can index the folders manually from Indexing Options.