If your titles aren't showing up in-game, here a a few common examples that could be the cause. Be sure the issue you have is still happening when Titles is the only active mod! If it's not, and if the cause is a conflict with another mod, any changes you make won't work!

Unlike Java (the programming language in which Minecraft is written in), json doesn't have a comment syntax. In Java, lines that start with "//", or are surrounded by "/* */", aren't read by the language compiler and are there just for humans to understand, often to leave themselves notes.

// Just prints the supplied string to the console
// jsons don't like these comments
public void someMethod1(String s) {
    System.out.print(s);
}

/*
   Just prints the supplied string to the console
   jsons don't like these comments either
*/
public void someMethod2(String s) {
    System.out.print(s);
}

The example json provided in the mod includes Java-like comments to help explain the needed json values for titles. However, if you copied the example json directly to your own titles, it'll include the commented lines which will cause Minecraft to stop processing the json, and thus not add the title to the game.

So just make sure all the lines that begin with "//" are removed from the title json.

If you are using commands to add or remove your custom titles, but don't see them in the list, try finding them by searching for "titles:[custom_title_name]". Each title added by their own mod are prefixed with their mod ID to prevent naming conflicts, i.e. botania:flower_pickup. To align with that strategy and other functions of the mod such as searching, the "titles:" prefix is added to all custom titles.

Vanilla Minecraft will only accept jsons that have file names with acceptable characters. They are a-z, A-Z, 0-9, and ".", "-", and "_". If it finds any other characters in the json's file name, it will ignore the entire json.

Each datapack requires a pack.mcmeta file to work. What it should look like, plus which "pack_format" it should be is explained here.

You might have another mod that interferes with Titles, with a common symptom being that your title doesn't show up next to your name in chat. One frequent culprit is the FTBUtilities mod distributed with most FTB modpacks and some third-party modpacks. Fortunately, it has a config option you can disable which will fix this:

# Adds chat colors/rank-specific syntax.
B:override_chat=true

Just set that value to false and it should work as expected.