It’s all about how you bid. I’m very junior, yet mange at least 90% bid satisfaction. From my experience some people’s bids are poorly thought out and would be contractually non compliant. Generically bidding as broadly as you can seems to work. Too many bids and it throws its toys out and gives up.
Having said that the new sodomiser does produce some interesting rosters, with days off often grouped at one end of the roster , meaning 3 weeks of only single days off at the other end.