Google not using HTTP IF MODIFIED SINCE?

[referer]

I read somewhere that Google sends HTTP_IF_MODIFIED_SINCE if they already know about a page.

My pages are already known by Google and sometimes they send the header. But in 90% of the cases they dont.

Why could that be? I haven't added a single page in one year...

[theBear]

JAWAG, because the content changes due to dynamic elements that are invoked to create the page.