Camellias need feeding, both in the soil and especially in pots and tubs. The best time is in the spring, as they finish flowering and start growing.
Any general feed will help, such as Growmore or pelleted chicken manure. This can be scattered on the soil surface.
Liquid feed mixed into your watering can is also useful, especially if the leaves are a bit yellow, when a feed with extra iron is best, eg Miracid. Use every few weeks between April and July.
Slow-release fertilizer granules eg Osmocote are the easiest and feed all season. Rake into the surface. The lumps, sold as Tablets, can be pushed down into the soil - ideal for tubs.