Most people who manage a lot of their contacts and friendships via WhatsApp will have a hierarchy: An unspoken ranking of who they want to be talking to or who they'll reply to first. They might shoot out a handful of messages when they want someone to talk to, start replying to those who respond quickly but then move on to other people higher up that hierarchy if they respond.
I'm sure most people have noticed that conversation with some people just stops, or there is someone who you hear from extremely sporadically, and usually for a brief period. I'm sorry to say that if that happens to you, you're not near the top of that person's list.
This isn't a new thing; it can happen in in-person friendships too, but it's harder and a lot more rude to leave someone 5th on your list at The Red Lion because you hear that someone in the top 3 is in The Crown.