There are a few reasons why you might be able to see the bra line through your shirt:
The fabric of your shirt may be too thin or sheer. If your shirt is made from a lightweight or translucent fabric, it may not provide enough coverage to hide the outline of your bra.
The color of your bra might be too dark. If you are wearing a dark-colored bra underneath a light-colored shirt, the contrast can create a visible bra line.
The fit of your shirt may be too tight. If your shirt is too tight or fitted, it can create a visible indentation where the band of your bra sits.
The style of your bra may not be ideal for the shirt you are wearing. Some bras have more seams or details that can create lines, which may show through certain types of shirts.
To reduce the visibility of your bra line, you can try wearing a different bra style, choosing a shirt with a thicker fabric or a pattern that helps camouflage the lines, or wearing an undershirt or camisole to add an extra layer of coverage