Curtain hardware problem

I have been attempting to install simple curtain hardware @ my daughters older house. The plaster above the windows will not hold the screws nor plastic anchors for the screws. Too many years of someone doing the same. Plus I keep hitting spots where I must be hitting metal or something about a tiny fraction into the plaster. Looking for a solution I thought maybe to put up a strip of wood & secure @ spots that will take an anchor/screw. Those places that will take hardware are out of range of the places where the hardware is to go. Anyone have an opinion or advice? Any of you have such a problem you solved? The wood putty did not work either.