Looks like this is a Schwintek Tripple Track system.
Found this article indicating a lot of people are having problems.
Thor slidout problems
Find the control box and check for error codes.
I extend my bedroom slides part way all the time when loading.
But I always hold the button for the extra 2-3 seconds when at the end of the travel so I always re sync the motors.
Sounds like you have a dead or disconected motor....not a sync problem.
A sync problem is when the box does not extend equally on both ends.