[pdftex] vertical line disappear

[Aleks Kleyn]

During last half year sometimes vertical line disappears from my pdf file.
when it disappeared from frame, I do not care. But when I draw diagram, it
is very important to show all arrows. In following example middle arow
disappeared

\documentclass{amsart}
\usepackage[cmtip,all]{xy}
\usepackage{framed}

\begin{document}
\[
\xymatrix
{
V_1\ar[r]^{f_1}&V_2\ar[r]^{f_2}&...\ar[r]^{f_{n-1}}&V_n\\
D_1\ar[r]^{f_1}\ar[u]|-{*}&D_2\ar[r]^{g_2}\ar[u]|-{*}&...\ar[r]^{g_{n-1}}&D_
n\ar[u]|-{*}
}
\]

\fbox{
(10.1.10)\ \ \ $f\circ(av)=(g\circ a)(f\circ v)$
}
\fbox{
(10.1.6)\ \ \ $g\circ(a+b)=g\circ a+g\circ b$
}
\end{document}

Here also vertical line before second frame disappears.

Thank you
Aleks Kleyn