## Specific issues
Here are some resources for specific issues you might encounter
## Optimizely app failing to load
If parts of the Optimizely app fail to load, you may need to ask your IT department to change firewall settings. For more information, see our help center article on [Troubleshooting: Optimizely application is blocked by a firewall, ad blocker, or content filter](🔗).
# Tools for troubleshooting
Here are some tools to help you identify issues if results are not showing as you expect.
The logger logs information about your experiments to help you with debugging.
You can customize the log level to the kind of information you want to track and specify where log information is sent. We have also developed default logger implementations for some SDKs.
## Error handler
The error handler is called when unknown experiment keys or event keys are referenced. You can provide your own custom error handler logic to standardize across your production environment.
If you notice that events are not showing up in Full Stack, check the following possibilities first:
UserIds may not be persisting. If a visitor's userID changes over the course of their visit, rebucketing can occur. This can have a direct effect on which events are fired. You will need to test your site end-to-end to ensure that userIDs persist for the entire duration of each user's visit.
The datafile may be out of date. If your datafile does not include all the entity keys—i.e., experiment_key, variation_key, etc—implemented in your site's code, this can have an impact on event firing as well. Ensure that your datafile is as fresh as possible.
## Notification listeners
Use [notification listeners](🔗) to trigger callbacks when certain actions are triggered in the SDK.