Hi
First of all, I am really sorry for lately response. I was spending most of my time this week to work on new version of DMS (and will be using it on my new website).
The issue you are talking about I believe it is because Paypal IPN is disabled on your Paypal account or your site has a plugin which blocks IPN messages.
if it is possible, please submit a support ticket and we will work together to get the issue sorted !
Regards,
Tuan