I thought I had the answer thinking you were using dbconnect1 but now I can see you are definitely using dbconnect2.
One of the main thing to check is to be sure your data isn't older than 2000 days as there is a default limit in the app out of the box.
Note: Be aware that DB Connect abides by the MAX_DAYS_AGO setting in the Splunk Enterprise props.conf file. When set, MAX_DAYS_AGO defines the number of days in the past, from the current date, that a date is considered valid. This means that any imported records with timestamps before today's date minus MAX_DAYS_AGO will not be indexed. The default setting for MAX_DAYS_AGO is 2000 days, so if you use DB Connect to consume database data that is older than that, you should increase that value in props.conf.
Secondly and the most likely is your input timestamp format. See below
input_timestamp_format = <value>
* optional
* specify the format of input timestamp column, in JavaSimpleDateString format.
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