Adding entities into Minecraft is a fairly easy process once you get a hang of it. The first time is usually a difficult, bug-prone process. This document contains some tips and tricks for fixing those dastardly bugs, as well as best practice information.
The best way to prevent invisible entity syndrome and other nasty bugs is by setting yourself the correct environment. You should review The Editor document for my editor recommendations. The most important part is getting a json-linter (or using an online tool).
The next thing you should do is turn on your content log. This can be done like:
Settings > Profile > Enable Content Log GUI. Then press
ctrl+h in-game to see any errors our output that might pop-up. Errors in the content log will show up every time you open the world, and also during gameplay if more errors occur.
warning: Errors are not cleared between runs, so it is possible the errors you see in the content log are old errors, from prior runs.
The content log is saved in
.txt format at:
You should also download the vanilla resource and behavior pack. You can find these here. You can compare against the vanilla files if you have any issues.
JSON-Schemas are a valuable tool for file validation. You can learn more about them here here.