I am wondering why my tooltip does not show after clicking my picturebox?

For example:

I hold my mouse over my picturebox, it shows the tooltip, I click the picturebox, I move my mouse away for a second or two, I bring it back, and nothing shows, even if I hold my mouse there for 10 seconds...

But if I hold my mouse over my other picturebox, the tooltip shows, and then when I hold my mouse over the picturebox that was not working, the tooltip shows for it?

Does anyone know why?

Thanks