Hello Tasos
First tell us do you have multipilot or solopilot on that truck for hydraulic commands.If is multipilot that I was see in most cases can you tell me did your hydraulic multipilot works ok?
If he is good and you dont have error on display it is important that you check wiring on hydraulic block and make sure that wiring is not defective or some electric valve is not working.If all what I write to you is ok than your hydraulic block is problem and you need to replace it or service.Probably some valve is stuck and hydraulic mix function.
Hope this helps to you.
good luck