When reading book 5, Solovet seems surprised Nerissa knew of Gregor's predicament because Nerissa's prophecies are so shifty...
Awhile after the last page, you realize Solovet didn't believe in prophecies, either. Like Ripred, she just used others' beliefs to her advantage. She had to save face by recanting her question, or admit her beliefs
Why did no one believe in Nerissa's prophecies despite making war decisions based on Sandwhich's?
Because most people didn't believe in Sandwhich's either, but went with the flow like Luxa
Because Sandwich was a great historic figure who founded their culture and was a great warrior. Nerissa is a frail young woman whose prophecies are vague and fragmented instead of epic and poetic.