ROBO R2 z axis auto go down, help !

I've been using R2 for 4 5 years and this week it has really annoying problem :
- When the filament breaking and filament did not pass the sensor, R2 will be stopped for us to change/ load the new filament again BUT the table did not stay at the recent printing position.
- The first days, it kept stopped position for few hours (up to 3 hours) until I figured out and changed the filament.
- Recently, it even went down immediately when R2 stopped. And of course I have to start again from beginning.

What is this problem ? Anyone have experiment with it please help me, thank you.