My most recent round of problems (similar to whatwhat you describe) was because the new batch of Witespy OSDs require more voltage. They may look like they're working, and they definitely load firmware and update and the lights flash, but no text will be overlaid without a full 5+ volts. Remember that just because you give a board 5 volts doesn't mean 5 volts comes out. Some is dumped through the regulator. Many boards operate at the very bare edge.
So at 4.9 or 5, I had a blank screen, no text. At 5.4 suddenly it came to life. The new batch are using a different regulator than the old batch.
One important tell is the telemetry light. It's yellow and flashes at a high rate of speed when data is being sent. Is this light flashing? It didn't flash for me at the lower voltage. When I upped the voltage, it began to flash and text happened.