[Plugin: Contact Form 7] Captchas disappearing?

I have Contact Form 7 (1.9.5.1) on WordPress 2.7.1 with ReallySimpleCAPTCHA 1.0.

It seems to work fine most of the time, but every so often, the Captcha image elements in the form stop working: the link to the image seems to break. If I edit the forms and generate fresh Captcha pairs, then everything seems to work OK again (for a while).

Can anybody suggest why this might be happening or where I should look to find out what’s going on?

Thanks!