Very odd. I've been using W&N Galeria flat matt varnish for the last few years. Usually it has excellent covering power and will send even the shiniest miniature dead matt.
I have had trouble with an old bottle, half-full, which hadn't been used for a year or two. This had a tendency to leave slightly shiny patches. Perhaps the mattness deteriorates after a while unused in the bottle... I chucked it away, bought a new one, and have never had the same problem again.
You definitely need to slap it on pretty generously though - not so that it gathers and pools in crevices, which is an invitation to the dreaded 'white bloom' - but certainly don't brush it out too thinly over the surface of the figure. It needs a good coating. (In part because it's a water-based, not polyurethane-based varnish, so it's not as hard and its protective qualities are less good and less durable. So if your figures are going to get a lot of handling, you want them to have a generous coat of this varnish).
To guarantee maximum mattness, you should store the bottle upside down and as undisturbed as possible. Don't shake it.
When you want to varnish a figure, start to unscrew the bottle top in its upside-down position, only turning it the right way up at the last moment.
Now if you look inside the lid, you'll see that the 'thick stuff' has collected there - a much higher concentration of matting medium to carrier fluid, than if you take the varnish straight from the bottle.
If you feel it's too thick for you to apply as varnish (it shouldn't be, but if you prefer to thin it slightly for ease of application) just mix a few drops of clean water into that thick varnish that has gathered in the upturned lid, and off you go.
Apply generously, and try not to brush it out too much.
Unless you've got a duff bottle, that should give you a very nice matt finish.
Other points to note:
1. W&N Galeria starts to go off within 30 seconds of brushing it on. Which is another good reason to apply quickly and liberally.
If you try to brush it out after a minute or two, you will end up with gluey lumps, which you then have to wash off / brush away off the figure using a lot of clean water.
2. Conversely, although it starts to skin over almost immediately, it also takes a long time to dry completely - 24 hours if left alone in a cool room.
I tend to put a lamp on figures I have varnished, or pop them into the airing cupboard for a couple of hours to give some extra heat to speed up the drying process.
The problem is, if you try to handle them when they initially appear dry and flat matt, after say an hour, the varnish will still be soft and the handling combined with natural grease from the fingertips, will put shiny spots on raised areas.
So resist temptation, and do not handle for several hours if fast-drying, or for a whole day if just allowing nature to take its course...

Good luck
