Dlls working and then not working

I have an issue with several softwares.

They use the same dlls.

Now sometimes the program loads up fine no issues.

Then the following day Il start windows, run the program and il get an error saying its had a problem defiining one of the functions in the DLL.

Now these arent COM dll's but most likely made using Pascal, C, Delphi or assembly code.

Now the question is why would this happen. Is the Dll having mood swings? or is there something running (or not running in background) that could cause this?

If I gave a link to DLL would that allow anyone to have a look and figure out why?