Today Waterman's Florida Blue ink surprised me during an exam at the University. My Hemisphere wasn't writing good and the feeling was like it was running out of ink. This was impossible because the day before I filled the pen. When I opened it, I saw that ink in the converter wasn't flowing freely and remained at the top of converter when I turned the pen upside-down. I didn't expect such surprise from Waterman inks, especially in Waterman pen. The ink may be four or five years old, but looks Ok without strange smells, changes in color etc. I used it some years ago and it became one of my favorite inks. The pen didn't have any problems before. I used it with Parker, Sheaffer, Waterman and Pelikan inks without any problems. The day before I changed the ink, blue Parker Quink was there and ink flow was excellent. I washed and dried it before filling it with Waterman ink. At least I learned not to do such experiments before exams.

What can be the problem?

I don't know what the problem is. But I also have a Hemisphere, with a convertor and Waterman black blue ink (I have cartridges with florida blue).

A few weeks ago I cleaned my pens (including the hemisphere) according to some tips I found on this board. I noticed that after that cleaning, the ink in the convertor flowed much more freely, instead of clinging to the sides.

Maybe if you cleaned your pen in a similar way, it would solve your problem.

The tips were quite straightforward: Clean the pen and converter thoroughly of any ink (by lots of flushing with clear water). Then make a solution of ammonia and water (some say 1:10, others say as much as 1:3, I made a solution of about 1:5, I don't think it matters that much) and fill the convertor and nib section with the solution (as if you were filling the pen with ink). then take out the convertor from the pen and let all the parts soak in the same solution for a few hours. After that, clean the pen thoroughly and make sure you flushed out all the ammonia. Let the pen dry and start using it again.
In my case, I saw a definate difference in how the ink acted in the convertor.

I'll give it a try. Hemisphere's nib section is easy to disassemble, so that's what I do when I clean the pen. Converter I usually clean with some water. I'll try ammonia solution.