I can't win rankeds but I win a LOT of normals. What should I do?

Hi. I am a main Shaco... But I heard that he isn't good for silver so I am learning Rengar atm. When I play, my team is good... in early... but in late... we are all ******. I will try to duo with my cousing(he is better than me) that is bronze because the same thing. I don't know what should I do because.. We win every normal we play.... Help please(need some tips that really help me not like buying wards/don't flame etc. We do buy wards and don't flame.. We push and do anything but I can do 1v5..)
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