+If some equations render correctly while others don't, you probably don't have
+AMS* packages for LaTeX installed. Most distributions of TeX come with AMS*.
+In Debian/Ubuntu AMS* is in tetex-extra package.
+To check if that is the problem you can try those two equations:
+ x + y
+ x \implies y
+The first uses only standard LaTeX, while the second uses symbol \implies from AMS*.
+If the first renders, but the second doesn't, you need to install AMS*.
+