Problem with Amazon SES. Emails not always sending

Hi. We've been having a problem for weeks with our Amazon SES service.

Our mailing application is known as Sendy and it never gives us problems. Is a self hosted web app integrated with Amazon SES. We've updated the last Sendy version and the issue persists.

The issue is the following: our mailing campaigns are not sending properly. We noticed that the Daily email usage is much lower than the total number of emails we send from Sendy (If we send 10.000, only 5 are being delivered. (Based on the Daily Email usage in Amazon SES log)

An important fact is that we see that the Complaint rate grows, and we have never had any problems with it.

Sometimes a campaign does send well, and the open rate is higher, but most of the time, it doesn't.

The only change we have made has been the migration of Sendy from one server to another, but with the same configuration. The IP changed, and therefore we added it in Amazon SES, in the section: "Email Receiving -> IP Address Filters".

¿Any idea?

We would greatly appreciate your help.

Thanks in advance.