Chances for rolling the "Bow Attacks fire an additional Arrow" on a bow
Solution 1:
Suffix Bow Attacks fire an additional Arrow (of Splintering) Requires Item Level 86
Any Bow with Item level 86 or higher can roll that modifier as a suffix. You don't need a shaper one, but a shaper influeced can roll that modifier. Check your item level, probably it's lower than 86. Otherwise, it's just bad luck.