Hello!
This issue, and similar issues, are known by the Devs, and being fixed currently.
The main thing we know is that it may be due to you previously attacking and killing the guardian O.R.Cs.
If you know when you did this, try to find that save and load it, if you do not; you may have to wait until it is fixed- perhaps in the next patch- which will hopefully allow them to respawn, like every other insect in the Park.
However, I'm no Dev, so I can't say this for certain. I'm going off of what they have said, though, so it should be accurate.