Sum subscript appears beside the symbol instead of below it in LaTeX

[Amok]

Hello, I need write this equation and here's the code:

\begin {displaymath}
\[
\frac{\sum_{s_i \in S_{access}^{site}}{{e^{-\beta E(s_i)}}}}{Q}
\]
\end {displaymath}

It is working well, the only problem being that the sum ''subscript'' is not directily underneath the sum symbol, but rather to the side of it (probably because it's too long). I was wondering if you could help me.

Thanks.

 

[nbo10]

$\displaystyle\sum_{i=0}$

works