This can often be resolved by following the steps below...
1. Close down Ableton, along with any other preferences which may be running in the background. Please also disconnect any other non essential USB devices. This should help to eliminate any issues/conflicts occurring where other applications may be trying to utilise the MIDI ports by default.
2. Re-open Ableton and delete any exiting manually mapped functions by going into the MIDI Learn mode. You should have a drop down on the left side where any manually mapped files are saved. Delete these for the time being; again this will help to eliminate any conflicts with mapping.
3. Go into your preferences again. Try changing either the Input or Output on the "Control Surface" to "None". Then, change it back to your APC controller. When you do this, Ableton should re-load the control surface mapping, as indicated by an orange message which appears briefly at the bottom of the screen:
If this does not help
1. Ensure that your APC controller is connected directly to your computer with no USB hubs or adapters in the middle of the connection.
2. If you are using a desktop PC, make sure that the USB cable is connected to a rear port.
3. Try replacing the USB cable.
4. Run a MIDI monitor test to determine whether or not this is a software or hardware issue.