Trial spawners not turning into ominous spawners

While having the trial omen effect, every trial spawner I approach has failed to turn into an ominous spawner. These are all naturally generated, I have not tampered with them, and they are all within line of sight through the full 14 block range. I had already gone through and beat all the spawners once before getting the long-term trial omen effect, though before that I had a 15 minute trial omen effect which worked on one spawner before going away as I was dillydalling in peaceful mode and only turning on normal when near a spawner and ready. Im not sure if beating the spawners beforehand has something to do with it but I thought it was worth mentioning.