Packages get bought when they become urgent
No procurement schedule, so everything is a spot price and every early-order saving has already gone.
What's happening?
Without a procurement schedule, packages surface when the programme makes them urgent, and urgent buying loses every negotiating lever at once: no time for competition, no time to level the returns, no time to question exclusions, and the tenderer can hear the deadline in your voice. Long lead-in items are worst - the price includes expediting, and sometimes the programme pays as well.
The schedule is not sophisticated: every package, the date its order must be placed for the lead-in to work, counted back from the programme, with enquiry and tender periods long enough for competition to function. Kept against the programme, it turns buying back into a sequence of decisions instead of a sequence of emergencies.
The solution
Every package bought on the date it should be, not the date it screams.