hook_validate not work with two modules working together

I am with a big problem.

I am using a module that puts a field of type city (autocomplete), the node types that I set in the configuration of the module. This module uses hook_form_alter to insert the form in node types that I chose. This module has a small validation to verify that the user entered the city.

The problem that is happening is that the validation of the node type, which is the form of the city, stop being implemented.

In summary is this:

I have the module "A", which generates a form by hook_form_alter;
Put this form to display on the node type "B";
The module "A" has the hook_validate with their validations;
The node type "B" has hook_validate with their validations;
When "A" and "B" are together, the hook_validate of "B" does not work any longer.

Someone help me?