Evoker not summoning vexes

My friend and I were triggering pillager raids today, on Normal difficulty so there were five waves and we could both get the achievement for killing a ravager. In later waves we were getting evokers spawning. There were steel traps popping up to bite us left and right. But the evokers never summoned a single vex. I saw them go into the summoning pose with the particle effects, but the vexes never appeared.