I was at Route 8 trying to thief a Sitrus Berry (don't ask) using my Butterfree with Compound Eyes. Thing is, none of the Trumbeak seemed to hold a Sitrus Berry despite the 20% chance for them to hold it. Butterfree is leading my party if you were wondering.
According to several Bulbapedia pages, there's nothing preventing wild Trumbeaks from holding Sitrus berries. So I'm pretty sure you just got unlucky and should keep searching. source source source source source source Useless probability facts: If an event has an 80% chance of happening (like a Trumbeak without a Sitrus berry) and happens...times in a row, then you have found the equivalent of... 37, one shiny Pokemon 45, one Pokemon with Pokerus source 475, tied votes in 9 US states source 1140, a perfect SAT if you guess every question source 9001, a focus blast that didn't miss source