Check your Operators Manual for details like this. You'll see it says to LEVEL FIRST, then put out your slides. Parking brake set. Engine running for both leveling and putting out the slides.
Make sure you hold down the slide extend/retract button all the way through the operations. Do not stop. And once the slide stops moving hold the button for another 15 to 30 seconds.
After I let go of the button, I push it again and hold it until I hear the motors stop and click. I will repeat that process until pushing the button no longer makes any noise from the motors. The goal is to hear both front and rear motors click in unison. This syncs to the two motors. I do it on every extend and retract operation.
Here's a video from WBGO illustrating the process. I find it's not necessary to do the rapid in/out to resync the motors. If I'm putting the slide out I hold the button down until it stops moving and then I hold it. After that's done, just push the button again in the same direction. You'll hear the motors move just a tiny bit and click. Similar to what he shows in the video. Sometimes I repeat this 2 or even 3 times until the motor make no noise at all when pushing the slide out button.