In general inner text of a DOM element is different from displayed on screen. Browsers normalize text upon rendering, but DOM nodes contain text as it is in HTML markup. For example a browser may show the text as
Hello UserName!
and the text of the DOM element can be
Hello UserName!
Take this fact into account when searching for an element using it's text value.
//span[.='Welcome UserName!']//span[normalize-space(.)='Welcome UserName!']