If no wall plugs were used then the lintel above the headrail is metal. You will have to drill a 3mm hole into the lintel at the point required for the bracket and then use a self tapping screw. In fact a number 8 quick thread screw will now do the job. this used to be my job , have fixed scores of headrails like this. The original screws are probably still Ok as they have to reach the lintel after going through the plaster and any void there may be. The other alternative to the top fix bracket is to use a right angled bracket that is fixed to the window frame. If in doubt contact the firm that put them up I am sure thay would help.