ᐅ Issue: Connecting strip lights in series.

Hello,
maybe someone can help me.
I have been assembling a Pax corner wardrobe with 4 carcasses. At the top, I clipped on the Striberg strips. Then I started connecting them.
First, I connected them from left to right, which resulted in only the last strip, the one directly connected to the power supply, lighting up. After that, I connected the left strip to the power supply and worked my way through the right side during the night. Now 3 strips light up, only the last one does not. Basically, it shouldn’t be a problem with the sequence. So, my question is:
Am I doing something wrong, or is it likely that a connection cable or a junction box is defective?
I would appreciate any answers. Thank you.

I think it is likely that something is broken.

Yes, tomorrow I will try to figure out which part it is by plugging it in and out.