Whatsapp: blue ticks don't always work

Dear all,

I noticed that I do not always receive the blue ticks on Whatsapp. For exemple, I have sent a message to a contact, I received the 2 blue ticks, but for the next message (to the same contact and only a few minutes after to previous one), no blue ticks. So I sent another message the next day (again to the same contact) and I received the 2 blue ticks. But the previous message still doesn't have the blue ticks.

My sister also had the same problem once (no blue ticks for one message), and actually the person didn't reveive it, although my sister had 2 grey ticks in front of the message.

Does anybody else has the same problem?

What does it mean? As the message before and after have been seen, it would make sense that the message in between also...

Looking forward to your suggestions,

Cheers,

Charlotte