1) Do you get any error/warn messages in a server log files when you press the button?
2) Put rich:messages on the page. Because you can have some errors in validation/conversion and they don't allow action to be executed.
3) It looks like you are using Seam. I don't have much experience with it so that just make sure that your bean ContractDeletor is defined properly for Seam.
I was not using the correct form tag within the modal.
I wrote <form> instead of <h:form>.
It does not work with the former one but it does with <h:form> and also with <a4j:form>.
Many thanks for your help.