Strange signal issue

as the tittle speaking , im having a very very strange issue on my note 3 , when i use my main sim card , i get a very low signal ; i have even managed to change it , without any further results , i then decided to use another sim card from another carrier , then i got full signal bars , i have tried to factory reset my phone , but without any results , the only thing i didnt do is flashing my device as i dont want to use lolipop , as it is impossible to go back into an older stock firmware , by the way i did not root my device and im not looking into it , im still not sure if its a hardware issue or software , i am really confused , hope someone could help me
thank you in advance
EDIT : i forgot to mention that when i use my main sim card into another phone i get no problem at all .