I have heard about this once before – it’s definitely an issue on the Photos / iCloud side. Because Nitro uses the Photo Library to do all the work (it doesn’t bypass any Apple code to work with the photo library), then once the edit is stored in the Photo Library, it’s up to Apple’s code to send it across. You might try the usual things – restart both devices, check for outages on Apple’s side, etc. It might also just be slow.