ECC always incorrect

Hi everybody...

I´m using this plugin in the registration form in my site, but always tell me the anti spam question is incorrect try again although it is right, I tried several times and always the same thing happens.... others captcha plugins don´t work in my site because i don´t use any componet to modified the registration form, i modified manually the fields in the php file, i put in the file the line code to create a new fields... is the same error whit this plugin or what can i do to fix it?

thanks