Sometimes an item published with a Creative Commons license prohibits certain uses. For instance there are CC licenses that prohibit adapting, translating, remixing, or otherwise modifying the item (No Derivatives, ND). There are also some licenses that might prevent you from combining them with different CC restrictions and require that they are always shared, even in a new form or material, under their original license (Share Alike, SA).
Linking from your main text or activity to an outside source is an easy and effective way to direct students to other materials, and it allows you to reference and use items that can't be combined without ignoring their licenses. This is also an effective solution for materials that aren't necessarily CC licensed but are publicly available, like a YouTube video or a newspaper article.
Reach out to the original license holder and see if they are willing to make an exception. After a CC license is applied, the rights holder can't alter the license to be more restrictive, but they are always allowed to grant exceptions if they decide they want to.
While CC licenses enable wider use, some make it more difficult or impossible to use them in a remix (a new creation made of multiple sources) or to translate, reformat, or otherwise alter.
This is possibly the easiest to immediately recognize and understand as preventing remixes or format changes. A license containing non-derivative terms means that you can share the item freely, but you cannot alter it in any way or combine it with other items.
Although this license does not prevent modification outright, it does require that any work resulting from its use be shared under an identical license. This prevents it from being combined with different share alike licenses. For instance if a book is published under a CC-BY-SA license and an article is published under a CC-BY-NC-SA license, they could not be combined because each requires that the resulting item be published under an identical license, and one is more restrictive and one is less.
Unless you plan to profit from your new creation, this isn't one to worry about.
Below is a test-your-knowledge quiz adapted from OERu.org's Open Education, Copyright and Open Licensing in a Digital World course (CC-BY-SA). To take it, look at the items in each question. Can these items be combined under their current licenses? And if so, what license can you publish your remix under?