Bay windows frequently suffer damp problems, so your description will sound quite familiar to a specialsit damp surveyor.
If the dampness is˙an isolated patch just below the window cill, then the likelihood is that it's condensation. This is˙quite common in a projecting bay where cold spots can form readily.
However if the dampness starts at skirting board level, it needs examining for the possibility of rising damp.
The other possibility with window bays is penetrating damp as often the brickwork is single skin rather than cavity construction, so you need to check the pointing, the window cill drips and the general sealing between the timber and masonry.
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