By convention, the core WordPress does not cause conflicts of any sort. The onus is on theme and plugin developers to not cause conflicts with WordPress. Unfortunately, that doesn’t always work.
If you don’t mind and are able, please post a link to your site. We can check to be sure the problem isn’t due to client side issues.
If the issue is confirmed to be server side, the next step is to narrow down the source of the conflict. It’ll be either your theme or a plugin. Deactivate all plugins and switch to a default twenty something theme. Everything should work fine, though you may need to reconfigure your menu due to the theme change. The original menu should be restored when you restore your theme. You don’t need to fully define the menu, just add enough to do a test.
While everything should work fine in the default state, WP does have some very obscure bugs. This confirms you’re not encountering one. Restore your theme and test again. If the problem returns, your theme is at fault. Otherwise, re-activate your plugins, one at a time, most important first, testing after each activation. When the problem returns, the last activated plugin is at fault.
Once the offending module is located, your options are to locate and fix the bug yourself, contact the author for resolution, do without the plugin, or find an alternative plugin.
