I have two theories for this, one more likely than the other. The less likely: the wolves were nearby, so Alice's vision was blocked. The more likely: Alice promised Edward not to look for Bella's future, therefore she didn't see Laurent about to attack Bella, and seeing Bella jumping off a cliff was pure coincidental. When Bella meets Alice at her house after she has been cliff diving, Alice tells her she promised Edward she wouldn't look out for Bella's future. Seeing Bella jumping from the cliff was an accident