The Lookup transformation in SSIS is case sensitive. "ABC" will not be matched to "abc". There is no option to change this behavior.
There are a few ways to get around this:
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change the case of your source & lookup columns using UPPER() or LOWER() T-SQL function, or an SSIS function in a derived column transform.
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Disable caching by setting "Enable Memory Restriction" to true on the advanced tab for the lookup transform. This will force SQL server to do the comparison rather than SSIS. This may cause OR improve performance problems depending on the situation.