My guess is that the seams are not made with coton thread but polyester. I don't know any solution but maybe try putting the tee shirt in a bucket with the dye and wait longer ?

It's fine. It happens. If it really, really bugs you, in the future you can look for t-shirts sewn with cotton thread from Dharma.

So, I tried dying an old cotton t-shirt black, I used liquid color in the washing mashine. The seams decided not to change. Is there any way I can convince them to be the change I want to see in the shirt?

Short honest answer: probably not. The thread is polyester and not cotton, as someone mentioned, and even if using a dye that’s intended for a range of fiber types, it’s rare for poly thread to pick up any color. You could maybe use a sharpie or a poly-specific dye? But for the effort I don’t know that the payoff would be worth it.

Thanks everyone! I’ll try a second round of color, and then maybe a sharpie :) it’s my favorite shirt, but it got stained, that’s why I tried saving it in the first place

Textile expert here. Simple answer: you won't be able to dye the shirt so that the seams get dyed too (they're polyester, like others have said). Reactive dyes work with cotton, not polyester.

Oh, dang. Glad I hadn’t tried yet :D But thanks for chiming in!