Can't post pictures to Facebook from the native photos app until on wifi

Hello. I have an iPhone 6 (software up to date)....over the last couple of months I've noticed that if i post a picture to Facebook from my iPhone photos app it doesn't post to Facebook straight away if on 3G/4G.....instead it holds onto it and as soon as i get onto wifi signal it will post it. If i was in my home on wifi and tried posting a picture then its immediately on Facebook. Very strange.

Ive checked the photos app setting and Facebook settings but no luck!

Can anyone help 🙂