Last Prayer Problem (spoiler alert)
If Donati died at the end, then the saint the woman prayed to got away with the earrings without answering the prayer. The saint answered the first prayer without an expensive gift; so maybe it resented the implication that a bribe is necessary in order for it to act.
Or maybe Donati lived...without other source material (like the novel on which the film is based) we can't really know.
Or maybe the prayer was for her husband...I believe you could interpret everything she says during the prayer very broadly (maybe)...yeah that could be a big stretch.