Applying paint issues

[Stixang]

I'm trying to paint our hallway. I have bought Dulux Diamond Matt. I have sugar soaped and washed walls, leaving over 24 hours before painting. I have started painting but find the paint is rolling off the walls. Same thing happens if I use a paintbrush so I now have clumps of paint on the walls.
The previous owners were super organised and told us all the paints they used before we moved in. They painted the walls with Dulux Realife Tough Matt Almond White.
Anyone have any ideas on how I can get the paint to stay on the walls? Thoughts and suggestions greatly received. Thank you

 

[Notadecor8ter]

Hi,

I've just applied this on my landing and had some weak areas. See https://www.diynot.com/diy/threads/hall-of-horrors.477096/

I've put two coats of matt white down as a base layer, this still showed some weak areas after the first coat, and then it was fine.

 

[Stixang]

Thank you Deasonuk. Back to the paint shop then

 

[Notadecor8ter]

Are you watering down the Diamond matt as well?