if you have no signs of damp it could be a couple of things,
one, if the house is oldish it will have a lime render coat under the plaster skim, over time it crumbles away,
or, moisture coming down the chimney might be coursing the salts to come out of the bricks which in turn will break down the plaster..
Either way needs to come off and start again.
It could be one of two problems.
1. Rising dampness. This can be present without visible dampness. If the plaster is original then it is likely to be fairly porous and therefore allow evaporation. This in itself will release salts from the wall and damage the plaster. I would most likely recommend the services of a damp proofing surveyor to inspect as if you replace plaster without treating the source you may block the evaporation and allow a build up of dampness.
2. Possibly sulphate Salts. When a chimney has been used for burning fossil fuels over many years. Soot containing sulphate will deposit within the chimney. When this is allowed to become damp, then will release sulphiric acid and will degrade the plaster and the mortar around the bricks. This is usually due to a build up of humidity caused by a lack of ventilation within the chimney flue. Either way it is best to be checked out by a dampness specialist to ensure that the correct course of action is followed.
Peter Barber CSSW.,CRDS.