Your placement seems about right. However it may be the amount of light they put off- perhaps the shades cut down on the light. How is your overhead lighting, because that is where most of the light should come from.....

Your sconces seemed to be placed in the right position. However, the shades and the rating (30 watts etc) might be the problem. Your fixture should have a sitcker in it indicating the maximum size bulb (watts) you can use in each fixture. Put in the max and change the shade for a less opaque one.

Your sconce shades should be linen and not paper so the light is translucent through the shade.

It looks like you have room for sconces with two maybe even three lights.

We have a pair of single light sconces flanking our bathroom mirror, there was no room for anything larger. We supplemented the light with a couple of recessed ceiling lights. It has worked out very nicely.

Love your mirrors.

I'm looking forward to getting sconces... with just overhead lights, when I look in the mirror, there are always shadows on my face.

I have three tube shaped sconces and they are perfect. No shadows anywhere. Mine are dimmable LEDs too.

I agree with the commenters here. Your placement looks good. It is ideal to have it put up for your (or an average of your family members) height, around eye level. You do not want to catch a glare from the bare bulb when standing at the mirror, and you want it at the correct height to cast as much light as possible on your face. Side lighting does wonders for bathrooms & mirror areas. If you put the fourth sconce up, make sure to look at a "mock-up" of it on the wall first. It might be tight when you put two sconces between those mirrors. (although 2 sconces per mirror is ideal for balance/proportion/light if you have the room)

But I also agree with the other advice from the pros above. Get as much light as possible out of them. Use the highest wattage bulb and shades that let a lot of light through. Play around with the color temperature of the bulb as well. The light cast by the bulb will interact with the other colors of the room, but overall a warmer color temperature usually makes us look better in the mirror.