Some mails are logged, some are not

I have been using Email Log for many months, and it is a great plugin- thanks!

However, i am noticing that some mails (even the same mail eg. new user created mail), is sometimes logged and others are not ie. a user created mail at 5.00 pm is logged, but another one of the same an hour later is not logged. And i am using Sendgrid API, and can see that the missing mails are being delivered in the Sendgrid dashboard.

Any suggestion why this may be happening?

thanks
Vivek