High opacity emulsion

Damp is a strong possibility however it could also be due to some sort of detergent or another fluid that has previously been on the wall which would make the paint react. I'd say the best solution would be to apply a zinsser primer/ stain block to the affected area before applying your chosen top coat

Your walls could have damp coming through? Or it could be because you aren’t back rolling it as your applying this paint can flash and leave streaks sometimes

After cutting in each wall, roll it horizontal then vertical. This should produce an even finish. I presume you are using Dulux Trade, and not the retail inferior version. If that dosnt work, then you will need to seal walIs. I personally prefer Johnstones or Crown Trade paints

If it's still not covering, try gloss oil based let it dry for 24hours then apply larst coat

This could possibly be caused by damp on the walls although it is more likely to be no back rolling enough or the paint not being trade paint, I would recommend using crowns covermatt emulsion and if that doesn’t work try rolling it with a stain block then painting over it again

Try and sand down the wall and use some zinsser primer then apply the main paint,

I always insist on using high opacity matt. I highly recommend it to every client on every job. the product i ALWAYS use is leyland super leytex high opacity brilliant white matt. it has NEVER failed me yet. even on brand new plaster in 99% of all the work i have done i can get a perfect finish in 2 coats without diluting 1st coat as a mist. ..in 1% of cases where there may be heavy pva staining in new plaster,, the most i have had to do is 3 coats and even then the 3rd coat has only been on the stubborn patch. i have never known it to flash. which brings me to the conclusion that your problem is contamination. if the product i have suggested won't do the trick then nothing will. cheapest option? lining paper. and then super leytex. hope this helps. good luck.