I play Syndra on TT often and I do very well, but I take her to SR, and I just get pooped on. Any ideas on why this is/ tips on how to play better?

Play somebody else.

I find Syndra a bit too stiff to play on SR

Syndra is great and since u played her in TT where even if 1 out of 3 members of the team is really good can carry the game ,u should realise why u can't play in SR
She is really good ap carry, can take out the enemy carries fast and stun the whole enemy team with a good q/e or q/r/e/q combo.. But in SR unfortunatelly 1 person is not enough to win the game... U need ur teammates to be of some use in order for you to shine.