I add these points,in same logic ways to explore:

1-the .h file concerned must be in every PRG, VCX and SCX (concerned).
A standard way to add the #include file into VCX class library is from menu Class - Include File... when the class is open in edit mode.
Such #include file is then visible to all classes in the library.

2-
* #define IVSSQLEXEC goDatabase.oDBAccess.SQLExec
test in code to confirm :
#IFNDEF IVSSQLEXEC
messagebox(" IVSSQLEXEC is not defined")
#ENDIF


3-The problem can be missing class compilation if you update something in the .h definition file.
when you change .h file contents then you have to compile all classes explicitly by command:
COMPILE CLASSLIB YourFile.VCX
If the classlib is a part of VFP project then you may just check the "Recompile all" checkbox before project build

4-Check the exact syntax of the constants .

waiting to your feed back.