Terraria Community Forums

That might be problematic - have you checked the Underground Jungle recently? Are you certain that you killed all three of the Mech Bosses?

I'm just not getting the message "the jungle grows restless" and I have defeated all mechbosses MULTIPLE times, including the Motherboard, and I haven't found any bulbs in the forest! (motherboard is a modded mechboss

Did you do this in a Pre-Hardmode world, by chance?

nope, but I got it into hardmode only recently.