i think i found the solution for this problem. everybody was talking about not working first, but working after x time. i first of all checked my general cache setting. nothing helped.
i am also using sh404SEF on this joomla and this component has got his own cache. in the SEF component I put the cache setting for Alphacontent on "nocache". this resolved the problem for me. i hope this is a help for you all and also a way to search for the developper to solve this problem permenantly.