I have had some issues with Robert Oster inks and hard starts. In particular, they don’t work well in my Lamy 2000 fine nib and Pilot Prera medium nib pens, whereas a Pilot Iroshizuku ink I bought recently seems to flow a lot better. It’s a shame, because I do like the colours; I might try them in my Kaweco sport pen (which has a wetter medium nib) and see if they flow better.