Why isn't my iPhone charging after a (store done) battery replacement?

Even though the battery read 100% when you received it from the store, it still might not have been actually charged up 100%. So if the store changes your battery again you still should put it on the charger the first chance you get and let it charge for at least 24 hours before starting regular usage and then always let the unit run till it runs out of power and then charge it again from low power. And never put the unit on the charger until it is low power. Many rechargeable batteries if you charge then when they have a high charge retain a memory and once they are plugged into a charging device for their first year of service think that they are low when they actually aren't. So best to only charge the battery when it says it is low on power only.