Your users might have installed a JavaScript blocking browser plugin/extension that uses the EasyList or similar list. This is especially likely if the number of users missing from Intercom is only a small and infrequently occurring percentage. 

While Intercom isn't an ad-provider, adblocking plugins (e.g. uBlock, Ghostery, Adblock, NoScript, and Firefox Privacy Browsing mode) offer options to block JavaScript that includes tracking functionality. 

Since Intercom communicates conversation detail as well as offering the ability to submit attributes and events on your Users' behavior, we are included in some of these secondary lists. Though there are many lists, the majority of Intercom blockages are caused by our inclusion in the EasyList list.

EasyList is an optional supplementary subscription that completely removes all forms of tracking from the internet, including web bugs, tracking scripts and information collectors, thereby protecting your personal data.

At Intercom, we respect customer privacy where requested so we don't argue against our inclusion here, but you can certainly request that your users disable plugins like this, or add an exception for your site for better performance.

If you believe you're missing user detail in Intercom for other reasons, feel free to get in touch to look into potential integration issues.