The main thing is don't call your dog with the command if you're not sure she'll come. Every time she ignores you, you're reinforcing that exact behavior. It's better to first work on the command in a safe, boring environment where there's nothing to distract her.
The thing is, you need a sequence: at home or in an empty area, you practice "come here" with a super-valuable reward (not just a treat, but something really tasty and unexpected for the dog). You do this in short bursts, several times a day - five minutes each. The dog needs to catch the pattern: hears the command → runs over → gets something amazing. Once it works nine times out of ten in a calm setting, you start making it harder: add small distracting factors, then walks in busier places.
On the street with an adult Labrador, it makes sense to keep a long leash for a while - not as punishment, but to physically control the situation while you're retraining her. And the key thing: call her rarely, but when you do - make absolutely sure she comes, even if you have to go over and grab her collar. Over time, the command will become more reliable.