I wonder how to label each equation in align environment? For example
\begin{align} \label{eq:lnnonspbb}
\lambda_i + \mu_i = 0 \\
\mu_i \xi_i = 0 \\
\lambda_i [y_i( w^T x_i + b) - 1 + \xi_i] = 0
\end{align}
only label the first equation and only the first equation can be referred later.
You can label each line separately, in your case:
\begin{align}
\lambda_i + \mu_i = 0 \label{eq:1}\\
\mu_i \xi_i = 0 \label{eq:2}\\
\lambda_i [y_i( w^T x_i + b) - 1 + \xi_i] = 0 \label{eq:3}
\end{align}
Note that this only works for AMS environments that are designed for multiple equations (as opposed to multiline single equations).