Need Advice on Pressure, Damage Output

So according to consensus and http://matchhistory.na.leagueoflegends.com/en/#match-history/NA1/203289894, my damage to champions is inconsistant (on champs that deal heavy damage), bordering on low. I also seem to be relying too much on team performance and not appling enough pressure. My mindset in games like these is to not take risks, to farm up my build, and to not screw myself, and in extension my team, over by not trying to force ganks or objective fights without ample backup or the target enemy champ's missing of health. Am I playing too safely or greedily to end games when I have the upper hand, or am I focusing too hard on objectives and not enough on killing the enemy team first? From what I gathered of what I was informed on when I asked about what I did wrong in my last four ranked games earlier today, knowing how to end games is what will get me climbing ranks more steadily. Also, any champions or flex picks (guys for multiple positions like WuKong or Lulu, etc) I can try getting the hang of to get me LP? I mained Vi and Sej in jg, Leona in support, and Sion in top, but the latter I haven't played much of lately and the former two are ALWAYS BANNED (Leona's also popular and hated in the ranked sphere). Everything else is a gimmicky pick that I haven't chain-practiced as much in Normals with as those three, especially Vi.