AmakawaHaruto It's probably because there is no functioning Controller within range, or no AI Module within range. (Read the part tooltips of the drone cores, Controller, and AI Module!)
Here is an example of a drone that does not have its own Controller or AI module. It needs another ship with a functional Controller or AI module that is within range to continuously have control.
Here is the carrier for it, which uses Controllers.
The Controller looks just like a Control Room, but with a smaller sensor array thing on top.
On this carrier, the Controllers are near the middle, to the left and right of the third drone down.
Hope this helps.
P.S. The original design for the carrier is not mine, it is based off of the ship called Carrier, under the name Mr. Star, in the community ships mod. The fighters carried by it are my own.
