Minecraft wiki entry for Creating_behavior_packs, associated text: Make sure that you've typed everything correctly. Typos and missing punctuation will cause unwanted behavior!
Make sure the .json file is in the correct folder and named correctly!
You should have a folder called entities that contains all your .json files
Make sure your entity has the correct identifier
Check the .json file in the Vanilla Minecraft pack if you aren't sure what it is supposed to be
Vanilla's .json files can be found in /Vanilla_Behavior_Pack/entities or downloading it through this link