Kerning not working for uppercase / smallcaps

I'm fixing a font to create GSUB and GPOS with FontForge on Linux Debian.
I have not touched anything concerning the metrics, apart from the symmetrical reduction of the bearing for the apostrophe.
As far as GSUB is concerned, I have filled in the order aalt lnum, tnum, pnum, onum, then smcp and finally liga.
In GPOS I created only one table:

'kern' Horizontal Kerning loopkup

and a single

'kern' subtable Horizontal Kerning loopkup 0-1

only for Latin. I put kerning for upper / uppercase and upper / lowercase and at end uppercase / small caps in this form:

-----------------------------------
A    | V W
     | V.sc W.sc
-------------------------
V W  | A
     | A.sc
-------------------------
F P | c e o
------------------------------------

and so on. However, even if I see correctly all pairs in FontForge preview, using LibreOffice with real smallcaps (and not fake ones) or XeLaTeX, all kerning work fine except uppercase / smallcaps. The space between thw glyphs is determined only by metric, and kerning ignored. I realize that here I gave you only a few information, but at present I've no idea of the reason of this trouble. Any idea? Thank youyou