I can't connect to my sim card

I just bought a new U Mobile sim card and tried to replace my old Digi sim card. When I tried to connect u mobile at my first slot sim card it cannot detect my sim but it work well with digi sim. I tried connect u mobile sim on 2th slot it also work well. How am I want to connect my sim in my 1st slot? Please help me me!! I tried reboot, re-insert sim but it not work please