This is not a theme isssue but a configuration issue. If this ever happens it is most likely due to low PHP configurations on your server and it can be easily remedied by upping the values. Check out our Requirements for Heli post.
All plugin updates, including the premium slider plugins (revolution slider, layer slider) are included with the new theme version. Since you purchased a theme that includes a license for plugins, we have to issue plugin updates when we issue theme updates. The only way to update the plugins without getting a theme update is to purchase the slider plugins individually.
Let's find our the issue come from with these easy steps:
Activate a new theme. If issue’s gone, it’s theme issue. Otherwise, come to the next step.
Deactivate all plugins. If issue’s gone, it’s plugin issue. You can inspect element to find out which plugin has problem. Otherwise, come to the next step.
Update WordPress with the latest version.
Contact to your host provider if all above steps can’t find the reason.
If you run into trouble when installing Heli, please check out articles for solutions. If it's not there, please contact us for support.