1. find a local place that has the capability to check if your key is transmitting an RF signal.
2. How careful were you when you opened the key? Many times circuits, relays, capacitors, and traces on the key's IC board get damaged when cutting into the key to replace the battery.
A forum member that does this type of work on the side has a "wall of shame" where owners end up sending him their keys after their self-repair failed. And he shows how things on the circuit board have been destroyed in the attempt.
Click following link to see "Wall of Shame" images...there's also contact info at the bottom of the page if you feel you'd like to use his services for repair. If you click around on the site...you may even find an email or phone number if you need to speak directly to him. It still may end up cheaper than buying a new key from BMW:
www.BMWGM5.com Key Fob Wall of Shame
here's some examples: