This behaviour seems as you should expect, with \uline{\hspace producing the same space as \hspace but underlined

\documentclass{article}
\usepackage{ulem}
\begin{document}
one two three one two three one two three one two three one two three one two three one two three one two three
one two three one two three one two three one two three one two three \uline{\hspace{5em}} %no underline showing
one two three one two three one two three one two three one two three \uline{\hspace{5em}} %no underline showing
one two three one two three one two three one two three one two three \hspace{5em}x1
one two three one two three one two three one two three one two three \uline{\hspace{5em}}x2
one two three one two three one two three one two three one two three \hspace*{5em}x3
one two three one two three one two three one two three one two three \uline{\hspace*{5em}}x4
\end{document}
Normally a space is discarded at a linebreak, so the left margin is preserved, so you see this happening for x1 and x2, \hspace* forces the space to be kept so you see the space in x3 and x4, underlined in the x4 case.
\uline{\phantom{\hspace{5em}}}or\uline{\hspace{5em}{}}– May 25 '16 at 13:35\uline{\hspace{5em}{}}is not working – user15964 May 25 '16 at 14:09\hspace{5em} Foowon't print a space of the given width and thenFoo(i.e. omitting the\ulinecommand) -- I assume it's the\hskipwhich underlies\hspace– May 25 '16 at 14:12\hspace*which works. So your new comment shows a new bug ofulem? – user15964 May 25 '16 at 14:17{}doesn't work (it worked, but under different circumstances – May 25 '16 at 14:34\hskip ...dies out at the end of a line, as I can confirm now. I assume, that this leads to an empty box at the end of a line and as such\ulinedoes nothing – May 25 '16 at 14:36