Its an acid loving plant and needs ericaceous soil, I would feed it with special ericaceous food, maybe even a dose of epsom salts

could it be sooty mould?

Overwatering is another possibility and the pot looks to be a bit on the small side.
As well as ericaceous compost an occasional specialist camellia feed might help with the balance of minerals required.

Also it will be much happier back in the ground where its colleague is.

Why do some children not thrive as they should? That's nature, and it's the same for all living things. You've probably got a runt so to speak.

I would take some cuttings from the healthy plant, that way they will share the same DNA and they will be an exact clone of the mother plant.

1.just as an update its in a builders bucket perforated at base its got plenty of room 2.it hasnt been overwatered its rained for most of the year and dosnt get watered only in summer ...3 it looks looks better now than when i dug it up out of the ground 4. the area it was dug up is a natural ericaceous area where rhodos grow 5. where i dug it up it was within 6ft of the other camelia that is doing fine 6. i have had a camelia in a container for 30 years and its fine only being transplanted once when old container collapsed

as usual myron is the voice of reason and on the right track ( and thank god he never left as he threatened to) and yes i think i have a runt i was thinking of taking it back but i havent got a receipt and its a long time since i bought it

To be fair Alistair, the yellowing leaves are signs of deficiency, iron and manganese deficiencies to be more specific. An ericacious feed would help with that so that was good advice. It is possible that the spot that you had the Camellia in the ground was too dry/poor. Rhoddies can also rape soil of nutrients. The fact that it has perked up in a pot and is now showing signs of stress (and nutrient deficiency) suggests that it is not a 'runt' but that you just had a rotten spot in the ground.

And sometimes a garden has a spot where nothing will grow (outside the back of a garage where the previous owner always cleaned his paintbrushes for example).

i take on board what you say and agree that the camelia does show signs of deficiency but like i said i have one in a pot for 30 years and often shows signs of the same but flowers well every year ...its seems it hasnt grown in 3 years that worries me more its sister is now over 4ft tall

What was the rootball like when you lifted the poor Camellia? I wonder if Vine weevil grubs or Cutworm reduced the roots and that set the plant back? They do not tend to attack Camellia but it is not unheard of.

I still think the pot (bucket) is too small, but if you moved it as rootball from the ground into the pot it may well be putting on root growth at the expense of top growth.
I'd check the roots and plant it up in a proper pot in ericaceous compost and top dress every year. Of course it would be better in the ground, ideally not where it is having to compete with established shrubs, and cosseted through the next year or two.

thank you for all your helpful replies and i will inspect it after its finished flowering this year and put it in its 3rd spot out in the garden this time i hope it will put on some growth

Camellias can sulk after being moved so don't expect a huge degree of growth for the next year.