Your question may get an answer if you include the OpenType code you are using. Please also note that most people are unfamiliar with Katakana, so further information about the issues you faced would also help.
A general remark: many applications still do not support OpenType, while others include just partial support. Full access to OT resources seem limited to InDesign, Illustrator and Quark, ALAIK. Anyway, if this feature works in other fonts, this shouldn't be the problem.
Another point is that FL Studio 5.2 is still a beta, so it is expected not to work perfectly. Have you tried to generate the font with FLS 5.0.x?