Hi There,
Thank you for writing in, do you have a custom portfolio template? Please switch to parent X theme and see if that resolves the issue. If it does then you know that it is on your child theme that is causing the issue.
You do also a testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Clear and deactivate any plugins feature your site might have while doing this test.
Let us know how it goes,
Cheers!