Pen won't start writing unless paper has already some wet ink on it

[Cassotto]

I bought a TWSBI Diamond 580 with a 1.1 nib some time ago. Apart from the fact that it looks more like a 1.5 nib to me, the pen originally came with a defective nib / feed (I don't know which one). I ordered a replacement, but I'm wondering what the problem with the original unit was.

 

The pen, when it writes, does so smoothly and with an inordinate amount of ink. I love wet writers, but this is too much for me. Anyway, I could put up with this, if there weren't another, more important problem. I've said "when it writes" because the pen won't start writing, no matter how much I try, on dry paper. The only way I can make it start is by turning the pen upside down (I mean with the feed pointing to the ceiling and the nib pointing to the paper), leaving a tiny ink mark on the paper, and then writing normally starting from that wet mark. As long as I use cursive and don't lift the pen off the paper, I can write forever. But at the moment I separate the nib from the paper and immediately lay it again on a dry spot, it won't write.

 

Why may this be so?