PC to iPhone and iPad

Hi guys , I'm hoping you can help..
When transferring photos from my PC to iPhone or iPad some of the photos just will not transfer . I have tried everything I can think of , does anyone know what is causing the problem . I'm pulling my hair out

I am presuming some are NEF and some are Jpeg.

Hi Scott , once transferred they are all JPEG , they all start as RAW though .
i have made a discovery however .. If I leave as the saved JPEG then some will not transfer .. If I open the JPEG back up in Photoshop and save to web , original size , 100% quality they transfer fine .. Ideas why I might have to do this on some photos ?

How are you transferring your photos? I use Dropbox and never have any problems.

I am confused. Are you syncing it thru iTunes for viewing in the Photo app in iPad/iPhone?

The photos are on my PC in a folder .. All I'm doing is transferring through iTunes when iPad is connected . It may transfer 30 fine but leave 4 that won't transfer . Just can't work out why some will not transfer onto iPad . They are all JPEGs