Manually setting TabIndex property?

I have a windows application in which I set the TabIndex property of TextBox controls manually. Based on the menu option that is selected, a specific panel, loaded with controls, will appear. I set the TabIndex property starting with 0. My problem is that when I enter data in a textbox and then hit the tab key, focus goes somewhere else as opposed to going to the next TextBox control based on it's TabIndex. I'm using VS 2012 and I'm sure nothing has changed regarding this but was wondering what could cause that since I immediately reset the TabIndex settings for a set of controls.

Thanks,