It could be a performance problem, where some part of your DB infrastructure simply can’t handle the number/pace of traffic from Ignition. That’s not terribly common unless you are trying to run the DB on the same server as Ignition.
More common is a misconfiguration somewhere in either Ignition or your DB that causes many SQL statements to fail, so the Store and Forward system puts the data in its data store to try again later. Which fails over and over again. You have to look in your gateway logs to find entries showing these kinds of failures. In some cases, you may have to turn on extra logging in your DB to see that side of the failures to figure out what is going wrong.