With the proliferation of 'push' notifications many people are becoming notification blind, or find them easy to ignore. Also many people have multiple demands on their time at any given moment or are multi tasking so giving attention to one thing or another will get sorted by a brain into priority order.
When you are on one end of an instant message (IM) conversation you do not know what is going on at the other and two individuals can often have different thoughts on IM ettiquette.
With some of my friends I can have a conversation over a period of days because we come back to it and respond when we are free, dipping in and out with no time based pressure on the other to respond. I appreciate these friends tremendously because they recognise that life is busy and are stable enough in our friendships to assume that silence does not mean we have stopped liking each other. Simply that the other is busy.
With other friends their etiquette is around larger meta information, informing me of what they are doing and how busy they are, almost timeboxing the conversation and wanting me to do the same. They want to get the information across and get replies then and there and if either situation changes we must update each other as to if we have to go and do something else, or if I am going to be away from messenger for any given period must update them with 'away from keyboard' or 'putting the kettle on' etc to manage their expectations over how long it will take me to reply because they will be waiting.
There are interactions between those two extremes also.
The difficulty comes when two people have different IM etiquette and neither adapts. They may assume things and attach meaning to the other person's actions when it is simply how they interact with their friends.
Or life got in the way and they have been called away from the conversation, or your conversation was simply filling idle time for them and they got busy. Not everyone will let you know that they are no longer paying you attention, they will just drop off the conversation making it easier to pick it up later.
I recommend you adapt to the others behaviour and interact with them in the way that they feel most comfortable in a medium that suits them best (many friends respond better to text than IM, or only want to interact face to face. but I know how to contact them in an emergency).
If they have not responded to your message then leave it, they have many other priorities that you do not know about. Don't nag or 'bump' that can be seen as rude by many especially if it is simply a casual conversation. Just pick up at a later date in your usual way of interacting.
If it is a casual conversation and they have missed your message then it doesn't matter. They have been distracted by something else, drawing their attention back is you choosing their priorities for them. They know how to use their phone/IM service, so I recommend moving your own attention elsewhere too for the time being