I use Magento 184.108.40.206 and I and use One Page Checkout Extension. Recently I have had an error where orders are not created within Magento despite successful payments. This happens for both PayPal payments and Credit Card. I also receive a Payment Transaction Failed Reminder emails (this would normally happen only if payment was not accepted/declined). This does not happen with each order - only some.
#1 Please enable log to see error.
To turn on the logging, you can navigate to System > Configuration > Developer > Log Settings and set the «Enabled» option to «Yes»
Don't forget to set full access permissions (777) to the following folder: /var/log, since that is the place where those log files are stored.
#2 Which one step checkout you used?