We've been noticing a trend over the last few months that typically around the 21-23 of the month several of our recurring contributions end up not being processed by iATS and then get stuck with a Pending status. Where are the first places that I should look to figure out the problem of why there is this pattern? We are using Civi 5.10.4 and iATS Extension 1.6.2.
First place to look is to ensure your cron is running properly. The verification is managed by two scheduled jobs: one to pull in the journal from iATSpayments.com and one to the verify (checks the journal entries to see if any of your contributions have been completed).