If the user is actually logged into your site, caching shouldn't be doing anything. Normally, caching is disabled for logged in users for many reasons.
If they aren't logged in, and you have have the System - Cache plugin enabled, that would likely be the cause. Unfortunately, the System - Cache plugin is a page caching plugin (the whole page is saved), and there's no way to 'opt-out' of that.. so if that's the cause, you'd have to disable that plugin.
Hope that helps, but if that's not the problem, let us know, and we can look into it more.
Thanks,
Alex