Hmmm, it sounds like you are reading social cues correctly. You notice that people are interested in the conversation at first and then seem to lose interest later on.

If these are dates you are describing, where you both seem to have a good time at first, and then the second date never happens even though you expected it -- that is very normal.

It's kind of natural to meet someone new, have a nice conversation, and then realize that you don't have all that much else to talk about (for friends or dates).