Not able to do a back up for iCloud

I have 4GB available space and the system says I don't have enough storage to do the back up, which is not true. There's enough space for what I have on the phone. It did happen that I run out of space when I was taking photos, but then I deleted old photos from my phone and from the iCloud and since then the back up doesn't work.

Until I make a back up I cannot download my newest photos from my phone.

What do I do (I am connected to WiFi)?

Thank in advance,

Petra