Agh! Why is my paint splitting?

[Playduh]

Touching up a small area of wall, same paint as used originally. Sanded down and cleaned area before I painted. Now as it dries the paint is splitting length ways.

What have I done wrong?

[newtscamander]

You need to water the first coat well down, otherwise the walls suck the moisture from the paint and it cracks as it dries.

[DailyMailYobos]

Did you wash the walls before painting to remove grease/dirt/grime?
Did you stir the paint before applying?

[Playduh]

Washed walls down and left to dry.

Then stirred paint. Possibly not enough.

Do I sand it down, clean and try again with thinner, better stirred paint this time?