i think that u should check  u'r struts.config.xml and
set the validation  tag to true...
if u used from a action and form  bean... u should do
same this for validation:
<action path="/edit" type="action.EditAction"
name="EditForm" validate="true">

but my problem  is only for validwhen, other validation  instructions
work right, and I had set validate="true" in strutsconfig file,
before.
I myself think maybe somthing in commons-validator.jar is wrong or
something like this.

When you can use struts  valdation that type of your form  bean be "DynaValidatorForm".

I think DynaValidatorForm correspond to DynaActionForm, but my
formbean is a kind of ActionForm, so I have used validatorForm. when
my validation  instructions are simple , for
example "required", "minlenghth" and "maxlength", "integer" and so
on, validation works fine. But when I want to define some validation
rules that they are complex a bit (for example comparing two fields
with each other by using "validWhen") it doesnt work  fine and it
ignores this complex rules  (I should emphesize it only ignores this
kind of validation rules, simple validation rules work fine yet).
The type of form  bean is validatorForm and in strutsconfig.xml I set
validation="true" and I set the plugin for validation-rules in
strutsconfig, but the problem  exists yet.

@first check  the version that u r using, bcuz latest version of
validator support validwhen.
Otherise u can try to use requiredif either.
I've some samples whenever u want just put on a message.