WhatsApp help

[BrownallJobCentre]

I sent 3 messages to the same person, first and last have two blue ticks but the middle one has stayed on 2 grey ticks which I think means sent and received by the other persons phone.

Why would the middle one stay grey when they’ve obviously read the last one?

They could have deleted your chat when the second message came in without reading it. And then when your third message came in, they read that one.
If you are in the main WhatsApp screen, you can select a chat to delete without opening the actual message itself. So it will stay as a grey tick.

I think if you pull down a message from the top of your phone screen to read it and select "mark as read" the ticks stay grey.