You are not specific about the errors
that you are getting, so it's hard to
diagnose your problem. Here are a
couple of ideas:

If you are defining a label in your
macro definition and are invoking the
macro more than once within your
source file, the assembler might generate a "Symbol Redefinition" error
if the label has not been declared to be "local".
For some assemblers, there is a "LOCAL"
directive to declare a specific label
as local. In other assemblers you can
declare a specific label prefix (e.g.,
"??") to be a default mechanism for
specifying labels as locals.

If you are defining a symbol constant
in your macro definition and are
invoking your macro more than once
within your source file, the assembler
might generate "Symbol Redefinition"
error if the symbol is not defined with
a "temporary" equate directive. It does
not matter whether the redefinition
applies the same value or a different
value with each invocation.
Some assemblers use an EQU directive to
define a symbol to be constant for the
duration of your source file. Symbols
defined in this manner cannot be
redefined. Some assemblers use the
equal sign ("=") to direct a temporary
definition for a symbol. Others use
"SET" (ref. Keil assembler and Peter
Dannegger's code examples). Still
others use "TEQ," for Temporary EQuate."
Symbols defined in these manners can
be redefined.
Maybe this information will, if you
haven't solved your problem already.
Chris