sorting problem - RadGrid GridTemplateColumn

my code that has problem sorting template columns:



the instruction says "the SortExpression value should match the data field you want to sort on (typically the field a control in the template is bound to)". I made sure this rule is applied but the problem remained. Then I removed the HeaderTemplate components, instead I added value to HeaderText property in the GridTemplateColumn level. This solved the problem, not sure why though.....

the new code looks like:
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