Try restoring it using iTunes. This may reset the software and get rid of any problems.
The way someone explained it to me (and it may make absolutely no sense) is that sometimes computers/phones get used to the parts inside them, so replacing a part will make it not know how to use the new part. Resetting it makes it learn how to use the new part, and can fix things.
This may be a myth, but sometimes the tiniest, weirdest details can fix problems like this.