unstable molecules

In her first appearance, Pym grafts antennae into Janet along with wings. Most subsequent stories don't depict or reference the antennae. They only reappear when she gets mutated during The Crossing. Was there ever a reason she stopped manifesting them?

Wasp did have antenna in early issues with Ant-Man. But Jan did not use her antenna to talk to the insect world much. Over time, she stopped using her antenna altogether.

Hank restored her antenna years later with a fresh batch of Pym Particles. You see, Janet still had those antenna cells, but they had atrophied from disuse. From there she started to show antenna again.

Once the Pyms' powers became natural circa Avengers #28 (and they no longer needed to expose themselves to new Pym Particles each size-change), Jan's antenna stopped manifesting. She occasionally used replacement tech, like a cybernetic collar from Avengers #167.

In West Coast Avengers #33-34, Hank explained that the cells for the antenna were still present in Jan's body...they just stopped growing when she shrank. He used his Pym Particle control as Doctor Pym to stimulate the cells, and Jan grew them on her own the next issue.

So, basically, Jan can have antenna or not when at wasp-size as she chooses these days.

More than likely she feels the antenna clash with her outfits. We all know how important fashion is to her.