We keep around 100,000 entries in the log database, and periodically remove older ones. If your system is really heavily used, it may be pruning logs too fast. It’s more likely though that the log viewer just didn’t fetch that far back: it only loads the most recent 500 entries by default.
To see more entries, scroll to the top of the page and click on the “Fetch older entries” link:
To narrow down what you’re looking for: all writes will be listed under the Audit logger, so searching for Audit in the log viewer should help you find what you’re looking for more quickly. That only searches the entries that you already have loaded in your viewer, so you may have to hit Fetch older entries a couple of times until you’re looking at the time span you’re interested in.
Alternatively, if you click the Download Logs button, that’ll get you a text file with all of the log entries currently in the logging database. You can open it in any text editor and search for the word Audit to find what you’re looking for.