possibly a bad earth. run a wire from the negative/earth on the battery to the negative/earth on each bulb. also using a spare bulb plug it into the socket and see if it is bright, if yes then you may have duff bulbs fitted.
Is this apparent only on dipped beam or main as well?
If its on both suspect a bad earth, but it would be unlikely both sides would develop a poor earth connection at the same time: if its only on dipped then suspect the dim dip fuse/relay/resistor. I would go for the second scenario first (I replied in the same order as markja and brooksy).