Would you happen to be using an HP600? Here is a blurb from Autodesk:
Issue
Arrowheads and text using TrueType fonts may be missing from plotted output on Hewlett Packard DesignJet 600 plotters. Lines with width may also not be plotted. This problem specifically occurs when AutoCAD 2000 is configured to use the HP DesignJet version 1.0 or 1.01 Windows optimized System printer driver.
Solution
Configure AutoCAD for the Internal HDI HP-GL/2 driver. Although this driver is slower, customers report that the plotted output is better.
To configure AutoCAD to use the HDI HP-GL/2 driver:
From the AutoCAD File menu > Plotter Manager.
In the Plotters window, click the Add-A-Plotter Wizard icon.
Choose either 'My Computer' or 'Network Plotter Server', depending if the plotter is attached to your computer or is accessible through a network.
From the Add Plotter-Plotter Model dialog box choose Hewlett Packard from the Manufacturer list.
Next choose HP DesignJet 600 from the Model list.
Continue through the configuration, supply the appropriate port, plotter name, and other settings until finished.
In the Plot command, select the HP DesignJet in the Name field on the Plot Device tab. Be sure the Plotter field now says "HP DesignJet HPGL/2 - by Autodesk".