Ending ligarure

Hello to all,

I am almost finished revising one of my script fonts on FontLab. There is an alternative ending "s" labeled "s.final.1". Under OpenTypeface window, I have it under "calt" feature, it reads like this:
 
ignore substitute s' @LETTER';
substitute s' by s.fina1;

This works fine.

BUT, I wanted to do the same with words ending with "os" so I created a gliph for this ligature called "o_s.fina1". Keep in mind there is also a regular ligature called "o_s" already set up to always subtitute "o" + "s" which works fine. But my final (o_s.fina1) ligature does not. I have set it up, also under calt, like this:
 
ignore substitute o s' @LETTER';
substitute o s' by o_s.fina1;

and as I mention, it does NOT work and I don't understand why.

I would appreciate any tips to make it work, please.

Kind regards