Hi Marcos,
Version 8.81 is now out of maintenance and there were issues with performance around alerts.
Is it possible you are triggering many alerts at the same time, so some are getting blocked in SQL & timing out?
The tables you need to check are OALR, OALT and OAOB ( and their sub tables). These may help to work out which have & haven't gone.
Are all your alerts sending internally, or are some going via emails? If one email address is incorrect and you have other recipients, there was a bug which meant none would send.
We have a customer on 83.81 PL10 we are about to upgrade, and had to turn most of the alerts off due to performance issues (100Gb database)