varchar fields that can't be text searched

I have 3 varchar columns:
Heading varchar(255)
LogText varchar(4096)
CallStack varchar(2048)

In the grid control, Heading is searchable using the right-click on the column header to get the "Text Filter" option. LogText and CallStack are not text filterable. Is this a limit because of the size of the varchar? If so, how do I override this?

Thank you,
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