It looks from the photo like it's a problem with the paint surface rather than the actual colour coverage. If it looks OK face on and the colour is even, then it's a problem with the texture you have given it rather than the coverage as such.
First off, forget the once stuff, with Dulux I'd always stick to Dulux Trade in either bog standard matt emulsion, or one of the fancier matt emulsions if you want a particular surface. Look for a Dulux Decorator Centre near you if there is one. I find them helpful if I ask for advice. They'll mix whatever Trade colour you want, even old ones that are not on the charts any more. Trade paint is thicker, so pros can thin it if they want. I don't thin paint, except for a mist coat (and for that I use Contract Emulsion anyway- never use PVA).
I also like clay paints, they go on like a dream, cost a bit but you need fewer coats and the surface is soft and matt and even. (But not hard-wearing and the colour range is limited.)
Next, the roller. The thicker the fur on the roller, the more textured the final surface. I like a shortish pile one for a flat matt surface on a very flat newly plastered wall, though not the very short pile ones, which are for gloss. Your decorator centre may be able to advise. If you're not experienced, use a roller tray. Put paint in the tray bottom. (I use a big ladle, it's less messy than pouring and the can lid doesn't get all painted up.) Roll the roller in the paint, roll it off on the slope, do this gently again and again until the roller is fully and evenly covered, then, again gently, roll off any excess so it doesn't slop when you put it on the wall, and then roll onto the wall. Roll evenly and don't be tempted to press hard because your roller is running out of paint and not covering- this will make it patchy- when you need more paint, just refill from the tray, rolling it gently on the tray slope to keep it even. I roll on angles, crossing over as I go, with a hand roller, but in strips with a pole roller. Youtube has examples to watch. I do ceilings and large walls with a roller on a pole, takes a bit of getting used to but is faster as you don't need ladders and you can use longer strokes. The main thing is, once you have rollered a patch, leave it alone. Don't go over it again until is has dried. If you roll over it when it has begun to dry, you will get a patchy surface. Always keep the edge(s) of the area you're working on 'wet'- keep working out from the edge(s) while it/they is/are still fresh and wet. I try to keep just one edge to work out from. And where you have to use a brush round things and edges, that's called 'cutting in'. Some people do it first, some do it after. I prefer to do it first as a rule, and then roller up to it when the edge is dry. Others prefer to do it as they go to keep all the edges wet. But you have to be very fast with the cutting in if you do it that way. All the best, you will get the hang of it if you keep at it, I'm sure.