Typical causes of this sort of problem are:
The use of cement based (modern) mortar in external wall repairs and pointing - this is especially bad for your walls. Cement mortars not only stop moisture moving freely through the wall and escaping into the air, they also encourage extra water into the wall through cracks that always form in cement. Cement mortars also force natural salts to crystallise in bricks adjacent to the mortar, causing the bricks to break up.
Cement (modern) rendering on the outside of the walls. Like cement mortars used for repairs and pointing, this sort of rendering cracks all over its surface - the size of the cracks depends on the quality of the render and skill of the builder but they're impossible to prevent. The cracks encourage extra water into the wall but the water cannot escape back out the same way because the render is impermeable. Even if cement render is not yet visibly cracked, it encourages condensation to form on the back as moisture from inside the house can no longer escape through the wall - this can even result in clay lump walls collapsing. Cement rendering a wall can be one of the quickest ways there is of ensuring a wall becomes very damp.
Modern plaster (usually pink in colour) on the walls - this too is relatively impermeable so prevents moisture passing through when applied straight onto solid walls. It may just form a thin layer over the original plaster, but in the worst cases, all the original plaster will have been removed and replaced with waterproof cement render covered with a thin layer of the pink plaster.
Modern paint or vinyl wallpaper used direct on solid walls (inside or outside in the case of paint). These too are mostly impermeable and again prevent moisture moving through the walls and escaping into the air.
An injected damp proof course - these are designed for use in dry walls and are incapable of working when installed in an existing damp wall. (They are either waxy creams that just form individual 'fingers' when injected, or they use chemicals dissolved in a water based solvent. The water based solvents must completely evaporate away after injection, to allow the chemicals to cure and form a water repellent coating - if a wall is damp, it never completely dries out and the injection process has just saturated it with even more water, therefore the chemicals never cure and form their coating.) As the injected damp prof courses never work, the walls are always replastered with a waterproof cement plaster to try and hide any damp for the duration of the guarantee. This just stops the natural moisture present from moving through the wall and escaping into the air, making any problems worse.
http://www.davidkinsey.co.uk/pages/damp.php
