Some hard edges

[Jermainesmit]

Hi guys! I have this problem when I am modeling shapes that have a curve. and then trying to get shapes out of it. After the booleans i can't seem to soften edges! Then it works and then it doesn't.
I looked around the internet a lot but I never found the problem that i was looking for.

[Juanpax]

You probably have a lot of Ngons (Multi-Sided Polygons). Can you show your wire frame?