There are two types of plot styles: color-dependent plot styles [CTB extension] and named plot styles [STB extension]. When plotting, you will only see plot styles of the type the drawing you are plotting is set to use. If you are not seeing your plot style file, and you are certain that it is in the plot style folder [check this in the Options dialog, on the Files tab], then you likely have a file that is set to use the other type of plot style files. Verify this by opening the Properties toolbar in Autodesk® Architectural Desktop 2004 or 2005 and examining the rightmost control, for Plot Style. With no active command, if it is grayed out and displaying “ByColor”, you are in a color-dependent plot style file. If it is not grayed out and you can click on the arrow to see a list of plot styles, you are in a named plot style file.
STB to CTB: Type CONVERTPSTYLES at the Command: prompt to convert your file to use color-dependent plot styles.
CTB to STB: You will need a conversion file to map your layers [by color] to a named plot style. If your office migrated from CTBs to STBs, you may already have one of these, based on your old CTB file; if you can access that, use it and skip the CONVERTCTB step that follows. Type CONVERTCTB at the Command: prompt and select a CTB file, to create a translation file. Then type CONVERTPSTYLES and select your translation file. NOTE: This will convert the file to use STB plot styles, but will assign plot styles to layers based on their color and the corresponding named plot style in the translation file. You will likely need to reassign plot styles, based on the STB file with which you wish to plot.
How to avoid the above mess? Set up template files - or use the out-of-the-box ones - that use your desired plot style type and make certain all new files are started using one of these templates. You can also set your plot style type preference in the Options dialog [Plotting tab], but this will only cover drawings started without a template ["from scratch"], which is generally not a good idea in Autodesk® Architectural Desktop, or when opening a pre-AutoCAD 2000 file, which was before the introduction of plot styles.