Captcha problem with Contact Form 7

Hi,
I have a problem with captcha code on my Contact Form 7: it doesn’t appear. Instead we have the text “captcha”.
When I save the form in admin interface, it resolve the problem but the next day whe have the same problem.
When i’m connected with admin user it works.
I have the Really Simple CAPTCHA plugin.
The code in my form is :
[captchac captcha-752 size:m]
[captchar captcha-752]

Thanks.