My Lab used to do the same thing. A wicked secret that Lab owners have never told you is that Labs LOVE to be chased

Don't play that game, b/c you can't win, and after you collapse and have a heart-attack, you Lab will come over and drown you, finishing you off.
She is engaged with you, but her method of playing is different than yours. She won't run off in these situations, unless you chase her. And if you don't chase her, she will come tease you... It's all a game to her.
In addition to some suggestions already posted:
1. When you Lab runs away, run in the opposite direction. She may chase you. You can then give a treat (or trade) and she'll drop the ball. Don't grab at the ball, b/c she will learn that game very quickly, and get better at keep away!
2. OR... I taught my Lab to stop nipping by using the Yelp method of Bite Inhibition. Afterwards, when he did something that I didn't like, I could Yelp, and he'd stop.... But, first he needed to be taught what Yelp means.
3. Related to #2, when she runs away, say "Oops" to provide a verbal marker, then dramatically walk into the house, leaving her alone (if that's safe). Come back out after 30 seconds, and give her another chance... It may take about 3 days for her to understand.
She wants to play with you, but doesn't yet understand the rules of the game. Once she understands, you'll have a much better interaction.