What causes contacts to get stuck at various wait stages? And how do I fix it?

I have 3 segments in my campaign.

Segment 1: is contacts added to Shared List A before the date 7/25/14

Segment 2: is contacts added to the same shared list before 8/2

Segment 3: is contacts again added to the same shared list before 8/15

The campaign is set up this way because it's for a limited time campaign. Segments 2 and 3 are not supposed to receive the first few emails Segment 1 receives.

Here's the problem: Some contacts (not all) are getting stuck in the wait stages for Segments 2 or 3 even though they should be in Segment 1 since the date 7/25/14 has not passed.

Any ideas why this is happening? How do I fix it?