Bedrooms are frequently prone to condensation, particularly if they are on an outside wall. Primarily the problem is˙they have little or no air circulation and get little in the way of heating. The lifestyle of your tenants could also be a factor - eg showers, washing, clothes drying etc. If this creates high volumes of moisture but ventilation is minimal,˙condensation will be inevitable and this will manifest itself on the coldest wall surfaces - which could well be the bedroom wardrobe.
If not already fitted, look at installing extract fans in the kitchen and bathroom to evacuate˙high humidity at the point of generation or a positive ventilation unit which will gently and continuously circulate warm dry air round the property. See the details of these products on our web site.˙