If you want a better chance of getting item drops, you need more Luck.
Charlotte has by far the best equipment for boosting Luck, as well as the
highest natural Luck stat. In order to get the most out of her, you should have
the following equipped:
Body: Princess Coat (+10), drops from #107 Nyx
Head: Princess Tiara (+7), in Forgotten City 4 squares below boss save room
Legs: Glass Shoes (+5), buy from shop for $5,000 after clearing 3 portraits
Acc1: Thief Ring, complete quest "King of Birds"
Acc2: Thief Ring, New Game+, quest reward glitch or buy from someone else
Magic: Luck Boost (+60), complete quest "Number of Fortune"
Have Charlotte as your main character and cast a fully charged Luck Boost for
+60 (regular only gives you +30). It doesn't matter which character actually
kills the enemy so long as Charlotte is your main character.
Also, you can get a permanent +50 Luck for beating Hard Mode. Good luck!