Problems hitting high notes on bari sax?

I've just started playing the bari sax, after having solid experience with the clarinet and alto sax. I'm understanding how to hit the low notes, but I can't hit the high notes well. When I get to C (in the top ledger lines) , D, and higher, I can't hit them at all. Although, I can hit those notes if I start with a lower note and slur my way up. Why is this?