First thing to check  is the file extension and name of the file to make sure
it is accepted by sharepoint.

Next I would check permissions and if anyone with admin status has altered
them in any way.

The file extensions are .ppt for PowerPoint and .jpeg for an employee
picture. These types have been checked in to these same folders in the
past without problems.

I did double check  the folder security to be sure something didn't get
changed and nothing did. The users still have author permissions.

I've had this problem in the past and a reboot has always fixed the bug.
I guess I ran out of luck there.
Any other ideas Steven?

Generally I've learned that when something works sometimes and doesn't work
other times that it is typically user error. I.E naming a file with a slash/
or '&" sign or something not recognized by the software. When you attempt to
upload you should get an error warning of some sort. Without being there to
see it myself I can't tell you what it is, but you might try copying the
error thread and putting it into the Google search engine to see what it
turns up. It really could be many different things. I know it's probably not
what you want to hear, but it may be of some help.