'Aligned' does not work for multiple equations

Question

I have four equations and the second one is not aligned with the others. I have tried 'aligned' in the code below but it doesn't work as expected.

    \begin{equation}
    \begin{aligned}
    \min_{w} \quad & \frac{\mu_{s_i}^Tw - r_f}{\sqrt{w^T\sum_{s_i} w}} 
     + \gamma w^T w\\
    \textrm{s.t.} \quad & 1^T w = 1\\
    & 0 \leq w_i \leq 0.5 \quad & \textrm{i = 1,2,..,7}   \\
    \end{aligned}
    \end{equation}
\begin{equation}
\begin{aligned}
\max_{w} \quad & \mu_{s_i}^Tw + \gamma w^T w\\
\textrm{s.t.} \quad & \sqrt{w^T\sum_{s_i} w} = \sigma_{s_i}\\
  & 1^T w = 1\\
  & 0 \leq w_i \leq 0.5 \quad & \textrm{i = 1,2,..,7}   \\
\end{aligned}
\end{equation}


\begin{equation}
\begin{aligned}
\min_{w} \quad & \sqrt{w^T\sum_{s_i} w} + \gamma w^T w\\
\textrm{s.t.} \quad & \mu_{s_i}^Tw = R_{s_i}\\
  & 1^T w = 1\\
  & 0 \leq w_i \leq 0.5 \quad & \textrm{i = 1,2,..,7}   \\
\end{aligned}
\end{equation}


\begin{equation}
\begin{aligned}
\min_{w} \quad & \sqrt{w^T\sum_{s_i} w} + \gamma w^T w\\
\textrm{s.t.} \quad & 1^T w = 1\\
  & 0 \leq w_i \leq 0.5 \quad & \textrm{i = 1,2,..,7}   \\
\end{aligned}
\end{equation}

Here is the screen shot:

Answer 1

Your equations are completely distinct and are not grouped in one environment. eqs 15, 17, 18 are aligned properly by chance because their widths are equal. here is a simple example (also there are many tricks and ways for doing this):

\begin{align}
&\begin{aligned}
\min_{w} \quad & \frac{\mu_{s_i}^Tw - r_f}{\sqrt{w^T\sum_{s_i} w}} 
+ \gamma w^T w\\
\textrm{s.t.} \quad & 1^T w = 1\\
& 0 \leq w_i \leq 0.5 \quad & \textrm{i = 1,2,..,7}   \\
\end{aligned}\\
&\begin{aligned}
\max_{w} \quad & \mu_{s_i}^Tw + \gamma w^T w\\
\textrm{s.t.} \quad & \sqrt{w^T\sum_{s_i} w} = \sigma_{s_i}\\
& 1^T w = 1\\
& 0 \leq w_i \leq 0.5 \quad & \textrm{i = 1,2,..,7}   \\
\end{aligned}
\end{align}