Oil reservoir oil doesnt return for multi system

Guys, I hope somebody can help me with this.

Why my oil in oil reservoir keep empty even though i already pour in quite a large amount of oil in my system.For example if today i pour in 20 liter of oil to my oil reservoir, it will become empty the next few days but the oil in my compressor is full. My multi system include freezer, cold room and island showcase cabinet. Im using refrigerant R-507.

Thanks for advicing.