undefined equation references

Hello,

if i try to reference equations 
begin{slide}{Title1}
\begin{equation} \label{eq1}
...
\end{equation}
\end{slide}

as in eq \ref{eq1}

i get an undefined reference.

However if i use overlays i can hack the problem.  

\overlays{1}{
\begin{slide}{Title2}
\begin{equation} \label{eq2}
...
\end{equation}
\end{slide}
}

then i can say ... as in \eq \ref{eq2} 
and it will number it properly. 

Is there something that i am missing?

Thanks,
paul



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