Subscribers Not Being Counted in Smart Campaign

Hello,

We created an email program inviting our social media followers to subscribe to our email list, with a link to the subscribe form. If they open the email AND fill out the form, the smart campaign should change that lead's program status to "subscribed."  But we have a few people who's program status didn't change to subscribed even though they received the email and subscribed through the form right away (minutes away). However, when I look into these people's profile, it doesn't show that they opened the email. It only shows that they received it and within minutes, they subscribed via the same form the email is linked to. It just seems like too much coincidence.

I asked Marketo support if there's any reason why Marketo wouldn't track an email open but they said no. They insist these leads found the form another way (even though they subscribed minutes from receiving our email).

Do any of you know if Marketo might not count an open in certain occasions (even when the lead does open the email)?