Math symbol doesn't work properly

Question

Something quite strange. Why I have as a result of the following formula:

\begin{equation}
\omega=\lambda_{1}\Sigma^{-1}\bf{1}+\lambda_{2}\Sigma^{-1}\mu
\end{equation}

a black square instead the sigma symbol? in the first part is ok. I am becoming a bit crazy trying to find out the error. I tried everything, to separate, to isolate, to write the formula with double $$ rather than \begin{equation}, packages loaded are the normal ones, etc... Any other symbol works perfectly but with \Sigma is quite weird, this is what I got no matter the way you try to write the formula.

Thanks for your support!

Answer 1

Using this code:

\begin{equation}
\omega=\lambda_{1}\Sigma^{-1} \mathbf{1}+\lambda_{2}\Sigma^{-1}\mu
\end{equation}

returns this output:

Only the \bf command was exchanged with \mathbf.