Issues with environment variables usually arise due to misconfiguration in `.env` files or improper variable usage. Verify the syntax and check for typos.

When environment variables fail to load in a Node.js application, the issue is often due to a misconfigured .env file or improper usage within the application code. First, ensure that the dotenv package (if used) is correctly initialized at the top of the entry file (require('dotenv').config();) to load environment variables before they’re accessed elsewhere in the app. In the .env file, verify that all variable names are correctly spelled, capitalized, and do not contain any unsupported characters or spaces around the = symbol, as these can prevent the variables from being parsed correctly. Also, check that the .env file is located in the root directory of your project, as some configurations expect this location by default. If using process.env to access variables, confirm that variable names match exactly; otherwise, the application may return undefined for improperly named variables. In a production environment, ensure environment variables are correctly set on the server or in deployment configuration files, as these might override local .env settings. Debugging by printing process.env and logging variable outputs can help verify that they’re correctly loaded, enabling the application to function as expected.