I think I’ve solved this now.
I went back through my workflow, and I found that if when I typed in the text of my engraving on my word processor, I shrank the page magnification slightly, then followed all my usual steps as stated above, the engraving came out just slightly bolder / blacker.
So at least I know my laser isn’t failing. I cleaned the laser with a bit of white spirit too. No muck came off, and the glass seemed to be spotless.
I think my laptop and word processor both updated recently, and that’s what caused this problem. But it’s sorted now. Thanks for your help gents. Zax I’ll definitely have a think about the steps you’ve recommended, but as I said, if it does what I need it to do then I’m happy to just leave things alone.