1. Existence of Sources
If your topic is obscure or extremely early, it might be hard to locate sources that exist or still exist.
2. Location and timing
If sources do exist, where do they exist and how fragile are they? Libraries that have the sources might not be willing to lend them or it might take a very long time for them to travel from the lending library to OWU. Is it possible you can find a digitized version instead?
3. Language barriers
If you want to examine a foreign event or peoples, unless you can read another language, you need to make sure enough English language primary sources exist that translate foreign sources or give an English perspective. The latter sometimes leads to a slanted view of a topic.
4. Ask a Librarian
It's our job to know the resources. There's also a good chance we've helped someone with your topic before. Paying us a visit at the beginning of your research can save a lot of time and frustration.