You know, sometimes people get wrapped up in their own worlds and they just don't think to initiate their side of the conversation. What this means, well, it can mean a lot of things. But, let's approach this from a different direction . . .
You make the effort to contact these people and that is to your credit. You are reaching out to them. Now, you cannot control what they do in return. If this one way exchange is really bothering you, then you might want to try a little experiment. For a couple of days or even a week, don't text or IM them to see what happens. By doing this, you are giving them the opportunity to reach out to you. Perhaps when you contact them, they are doing something and they don't have the time right at that moment to engage in an exchange. If you do not hear from them and you want, perhaps you need to go directly to the issue and ask them why they do not contact you first. You will have to be prepared to hear their answer and it may not be the answer that you want. The only way you are going to know is by asking . . .
What with text/IM it is easy to get overwhelmed with incoming messages and if nothing momentous has changed, many people just don't follow up . . . if you are worried whether these folks are your friends, do ask. Whatever the answer is will help you to determine your next steps . . . from what you say, you are a really good person trying to keep friendships going, but sometimes friendships have an expiry date but all this does will be to encourage you to make new friends . . .
Let us know how things go for you but remember that every person is unique and their lives change . . .this does not mean that you are any less valued . . . only that you are a good friend who is taking the time to ensure that the friendship is good for you and for the other person . . .take care, kc