Mophie Air vs Plus for iPhone 4S?

I just got the Mophie Air case a few days ago for my iPhone 4S. I used it recently, & it only charged it from 10% to 37%. Should I return it, & exchange it for the plus? Do you have the plus or air? How much is the air supposed to charge it back up to compared to the plus?

The iPhone 4S has a 1400 mAh battery. The Juice Pack Air is 1500 mAh. The Juice Pack Plus is 2000 mAh.

The biggest question is what you were doing with your phone while charging? Apple's default settings such as screen brightness burn more power while plugged in, so if you were using your phone, the phone actually consumed more power than it was storing on the battery while plugged in. Even if the phone was just on, it was still consuming power while the Juice Pack was switched on.

The second question - and probably the more relevant question to long-term performance - is where did you buy your Juice Pack? If you bought the Juice Pack from Amazon or eBay, you probably bought a counterfeit product. You usually get what you pay for, and counterfeits have no reason to meet the standards of the authentic product because they don't have a reputation at risk. Counterfeit manufacturers will often include smaller batteries or inferior electronic components because consumers can't read the battery pack from the outside.