A good way to think about primary sources is to think of them as having their own life histories. They have to have survived from the time when they were created until now. For us to be able to find and use them, they also often have to go through several other steps. This is the primary source lifecycle.
The source has to be created in a tangible form, either physical or digital.
An institution or individual has to decide to collect the source. Technology has to exist to collect the source.
The institution or individual has to decide to preserve the source. Technology has to exist to preserve the source.
The source needs to be recorded in some way so that it is findable. This also usually entails the creation of some sort of system for finding, like a list, database, or catalog. An item that has no record of its existence isn't findable.
An institution or individual has to decide to digitize and host the source. There are costs to digitizing the material, creating a system for hosting and making the source findable, and for ongoing server/hosting costs.
All images via Flickr Commons.
Disruptions in the Lifecycle
The primary source lifecycle can be interrupted, complicated, or ended at any time.
There is no technology for collecting or preserving the source.
There is a natural (e.g. fire, flood) or human (e.g. war) disaster that destroys the source. This applies to digital sources as well, which have a physical presence in the form of servers.
There is no record of the source or no system for finding the record.
There is no money for, labor for, or interest in collecting, preserving, recording, or digitizing the source.
Even if the primary source has successfully made it through this lifecycle, it might not be accessible to you.
It might still be in copyright. Materials published prior to 1924, as well as some other categories of materials, are what we call public domain, but materials published after that are still owned by someone. They cannot be freely distributed or digitized.
It might be subject to use restrictions. Some people or organizations donate, create, or digitize materials, but put restrictions on who may use them and for what purposes. For example, scans of the census are not available for dates after 1940 because the people listed might still be alive. Military records are similarly restricted.
Digitized primary sources might have been digitized by a commercial entity that sells primary source databases to universities and colleges. This is true of many of the primary source databases at Georgetown. Similarly, if the primary source is physical, the institution that owns it might not be open to everyone. Or it might only be held by one institution that isn't accessible due to location.
If the lifecycle has been broken or one of these other barriers is present
The primary source might not exist.
The primary source might not be accessible, easily or at all.