captcha error

both WordpPress and FormMaker Pro are up-to-date but a couple of my clients are getting “Error, incorrect Security code.” when trying to email from form. I’m using the wd-captcha because I can no longer get the Google based one to work (we were getting too much spam). Any ideas as to what is wrong?
Vickie Norris
vickie@webistree.com

https://wordpress.org/plugins/form-maker/