Hello there,
You are correct as it is very difficult for troubleshooting an issue without knowing the root cause.
This article explains how to use a type of function called an aggregate function to sum the data in a query result set. This article also briefly explains how to use other aggregate functions, such as COUNT and AVG to count or average the values in a result set. In addition, this article explains how to use the Total Row, a feature in Access that you use to sum data without having to alter the design of your queries. https://support.microsoft.com/en-us/office/sum-data-by-using-a-query-430a669b-e7fd-4c4b-b154-8c8dbbe41c8a
You can also use Microsft tools to find if there is any process that is interfering with the MS access tool.
Process Monitor is an advanced monitoring tool for Windows that shows real-time file system, Registry and process/thread activity. You can get the tool from here https://docs.microsoft.com/en-us/sysinternals/downloads/procmon
System Monitor (Sysmon) is a Windows system service and device driver that, once installed on a system, remains resident across system reboots to monitor and log system activity to the Windows event log. You can get the tool from here https://docs.microsoft.com/en-us/sysinternals/downloads/sysmon
Similar discussion here https://answers.microsoft.com/en-us/msoffice/forum/all/totals-function-no-working-in-access-query/e22e603c-f36d-47e8-83c2-54a6cecfc7c2
Hope this resolves your Query !!
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