Paid Memberships Pro plugin problem with sign up

Hi there
I am running WordPress 3.8.4 (this was not an option in the dropdown box).

Anyway, I have installed the free version of Paid Memberships Pro onto our site

The plugin has been running fine for the past few months. However, on two occasions now, I have been told that new members have tried to sign up – they went through the sign up process and everything seemed fine. However, no record of any payment or registration from this person was reflected in the members list on the website and they were not sent a confirmation email or a paypal receipt (I am not sure if the money has been taken from their account). I also did not receive a notification email to say we have a new member.

I checked the plugin and saw that it needed an urgent update last night (something to do with a security risk), so I updated the plugin and am no longer receiving this warning message. Do you think this could be something to do with the problem?

Other members have signed up ok with the last one subscribing on 10th November.

It seems to happen sporadically – this has happened twice now and I am worried that some people might be trying to sign up but the same problem is happening with them.

Any advice would be appreciated.

Also any advice on how to make Paid Memberships Pro a completely secure payment gateway to avoid anyone hacking us would be appreciated.

Thanks