This is almost certainly a condensation problem and if so, a very typical scenario. The reason is that wardrobes get very little air circulation and if they're situated against an outside wall, that wall will be a colder surface which is likely to result in˙the formation of condensation. And condensation frequently leads to mould growth. In all probability your clothing is at risk from the dampness as well.
The answer is to increase air flow in and around the wardrobes. So˙leave the doors open more often or fit˙ventilation panels. And keep the bedroom warmer if possible because warm air holds more moisture.
If the problem is really serious rather than a minor irritant, the best solution we can recommend would be to fit in the loft a positive input ventilation unit - which operates exactly as its name implies, by quietly pushing warm air from the loft round the property to eliminate cold spots and stop condensation (and mould growth) from forming.
˙