Roller painting tips

I am painting a full house with white emulsion. I've watched a few videos and seen the V or N roller method. Problem is that in the videos they seem to load the roller then go up and diagonal and back roll over 3 roller widths or more. I've tried this but the roller seems to dry out after I completing the N and theres not enough paint to back roll and spread the paint top to bottom.

I thought I may have been putting too much pressure on the roller so I'm deliberately going a bit lighter now but it's still pretty much the same. I've resorted to painting roughly 1m x 1m squares at a time and this seems to work well but obviously taking a lot longer.

I'm using Leyland trade paint which had plenty of positive reviews. Still got several rooms left to do so any advice is appreciated.