To better explain the reasons behind this, let's take the following example. Say you have the same keyword twice in a campaign, though in different ad groups, and they differ with, for example, only one being capitalized.

When we do the search for the search term in Google Ads, the system will do a case-sensitive search, and since the keyword is in a different case, and the search term is in a different case, the system won't be able to find it. 

If this happens, it is usually because the suggestion you tried applying in the Traffic Sculptor tool is already present as a positive keyword in your account. As a result, you can't add this keyword as an exact match negative, as it would result in conflicting keywords. You can read more about this here.