Cannot Edit Equations in Word 97 documen

When double-click on an equation in Word 97, get the error “Word cannot edit the Microsoft Equation 2.0” on some of them, or “Word cannot edit the Microsoft Equation 3.0” on others. Since some workstations running Word 97 can edit the equations, andothers cannot, it is not a problem with the equation in the document. I have already tried uninstalling and reinstalling on the workstations that cannot edit the equations. Word 97 running is part of Office 97 suite running on Windows 98 workstations.