Battery not working after replaced with a new one!

HI,

i already twice change a battery for my iphone 4s.

last time when i charged my phone, instead increasing the percentage but decrease.

so i went to the store and look for the problem, the guy said that there's a battery problem, then he changed to the new one.

but after apply the new battery, still the same problem happen, so the phone totally died because not connect to the charger.

i went back to the store and re-check it again , but the guy give up and said don't know whats wrong. he already re-install another new battery but not working, im not sure if he check the charging port but according to him its not that also,

but i dont want to give up. is there anything i should do by myself instead go to the store ? anyone have a same problem also?

thank

cheers