Well, I i'm not dissing or anything, but if you're Silver 5, you're not playing near perfectly. Silver is still basically Bronze, but with 4 wards on the map. Anywho, you're putting your own self at risk if you feel like your teammates are going to be like this most of the time and still pick Nasus. You could try picking someone else that has better early-mid game and help your team out more, to be safer. Not saying Nasus never works, he's a beast. If you still wanna play Nasus(and you seem to be pretty decent with him, no real reason not to), just gotta accept the fact people will 4v5, intentionally or unintentionally.
tl;dr
don't blame team for losing
try to focus on yourself more and maybe then you'll actually rank up
this too, working on it myself
people should play olaf more, if you build lifesteal for him hes so fun
I've actually been considering picking either him or Vi up, for jungling. There's also Lucian but eeeeeh.