Finally, I have to say that the form is working perfectly if I fill all the fields so the problem starts if I left any empty field... those fields are not validated!!
Thanks a lot one more time for your support and apologies for the code, it is in Spanish...
So looking at the server-side markup is not helpful; to you, or to us.
Do a View Source and see what the browser sees.
You gave me the key for making it works!! Basically, after viewing the code from the browser I could identify the real ID of the different resources of my form. Give that ID I was able to check if the field was empty or not so now it works nicely!!