I suspect that the wall is reflecting heat and sunlight and making life difficult.

My camellia sasanquas (they are different varieties; no sure if yours are all the same variety or not) have different growth rates too.

My planters' soil may go down a little due to watering and settling. Or if the soil is not kept evenly moist (so the soil dries too much) then the soil separates from edges of the pot.

You should be able to take them out to add more soil but, if you can, I would do it when they are somewhat dormant. So not in the spring, summer or early fall months for example.

If you see that roots are circling, take action now.

Camellias like shade. Your site looks too sunny , hot , and dry. I would put them elsewhere and find a plant better suited to pots in sun.

Thank you all for the tips,

I decided that 2 out of the 5 looked a bit sad so i removed them and placed Chinese Dwarf in the pots so that i get a bit of cover over the wall and this will break up the remaining camellia's making them more of a feature within the Chinese Dwarfs...

I am still unsure as to why the uneven growth rate based on the following,

1 : Pot surrounds made of 75mm Hebal Block -Insulated Panel and then rendered ( This would make it cool in the soil and it was with the 60mm mulch on top... Also the soil was pretty much perfectly damp / wet )

2 : The soil that was used was ( Osmocote Professional Rose Gardenia Azalea & Camellia Mix )

3 : I removed the decorative wood chip mulch and took some pics of the soil so that you can see.. ( sorry for the chips still present but it would of taken all night otherwise ).

4 : I could still see evidence of the Osmocote slow release fertiliser pellets / balls in the soil.

The root system was approx 300mm diameter and 250mm deep and the roots were bendy and healthy looking.

My guess would be that, all things being equal, some had a smaller root system and grew less than the others. Reflected heat (from the wall) and heat released at night by the rocks may have also prevented the camellias with smaller root systems from thriving more.