There seems to be an issue with the Robo Security Guards in particular, where you cannot install mods. You select the mod to be installed, and it never actually installs it. The guard gets distracted doing something else (e.g. fixing haywire janitors), and never gets the mod installed.

It is worth mentioning that my museum currently only has 1 x Level 2 Robo Mod-Stop.

Others appear to be having this issue too:
https://www.reddit.com/r/TwoPointMuseum/comments/1jhcbb2/upgrading_robots/
https://steamcommunity.com/app/2185060/discussions/0/594018181914341466/
https://www.reddit.com/r/TwoPointMuseum/comments/1jhypx2/what_to_do_if_reobots_dont_go_to_mod_themselves/

Suggestion: You should be able to drop the robots on the Robo Mod-Stop, like you can with staff on the training room.

Others have mentioned a workaround of unticking all of the robo guard's job responsibilities - I have yet to try this.