Hi June, I presume you have tried split grade printing in combination with the flashing?
Another technique to try is water bath development. Place a try of water beside your developing tray at the same temperature. Put the paper in the developer as normal and when the blacks start to emerge, transfer the print to the water bath by gently sliding it in without agitation.
The developer will quickly exhaust in the shadow areas but continue to develop the highlights. If you decide the blacks are not dark enough, you can put the print back into the developer for a short while and then back into the water bath. This can be done along with flashing and split grade printing.