Web Objects: Refuse to load

I’m trying to use Web Objects in my current Captivate project (Captivate 2019). Some webpages (such as a google drive doc) have no problem loading, while others (such as a slack channel) refuse to load when being previewed (in browser). They appear find when working with them in Captivate. I can’t find any details on why this is the case–any help is much appreciated!