So going off the WhatsApp FAQ, a single check means the message was sent from my device to their server. A double check means it was pushed to the other device. At least 50% of the time, I get delays 2nd check. Now I cannot say for sure they are getting delayed to the recipient phone or I'm simply not getting the verification immediately. Regardless, I have had this issue through multiple updates. FWIW, when this happens we are both in great coverage areas, because alot of the time its during work hours.
Often I won't get notifications or any indication on the icon. Then when I open the app, I get the vibration showing I have a new message. Then I see the message was sent some time ago.