tabbing issue at the DataTemplate

I am having tabbing issue for a DataTemplate within a Grid in a WPF program.

The program will automatically add a user control into a Grid. The user control has a DataTemplate, which has several textbox in a row.

The current situation is: I can add multiple rows into the above Grid. For the 1st row, tabbing each textbox on this row is good. But the cursor does not go to the 1st textbox at the 2nd row after tabbing the last textbox at the 1st row.

Any ideas about it? Thanks.