Smaller Brush size does not paint when painting texture materials onto Landscape

Development World Creation
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1

When I paint texture onto my landscape with a brush size of f.e 100, it paints perfectly.

However when I change the brush size down to 10 or lower it will not paint at all. Why?

I am using megascans textures from quixel bridge and default landscape mesh created in ue5 landscape tool.

thanks for help