This is all perfectly normal puppy behavior.
The "leave it" command should be used as a warning...for when you see them eying an object or looking at an item. The "drop it" command should be used when they already have something in their mouth. Keep your pockets stuffed with good treats...and show the puppy the good treat...put it near their nose...give the "drop it" command and most times the puppy will drop what they have in their mouth to get the treat. You have to do this numerous times...and always have a treat to trade for an item . Lots of praise too.
Keep puppy on leash while out walking and don't let her slow down to sniff too much if there is an object on the ground that may interest her.
Gina