Ya I think that is weird myself, just because I never post all of the sudden I'm not allowed to vote?! Is that the reason or is it something else? And like PaigeAnn, I didn't have a problem last month.
Only members can vote. There is logic behind this, let me explain.
Becoming a member from a junior member takes some effort. You need to prove that you have dedicated yourself enough to the site to either reply to posts, offer advice, etc.
This deters a single user from logging in with multiple accounts and voting for their cham, or getting their friends to do so.
Hope this sheds some light. This doesn't mean you're trying to cheat, but there are some people out there that find it worthwhile to do so.