PEAR Cache - doesn´t create Graphic-Cachefiles ?

Hi, i´ve a urgent problem to fix: at the moment i try to use the PEAR Cache - especially the Graphic Cache. I´ve done the integration in my button script - it works fine so far. That means no error messages are displayed. Also it creates in the Cache Directory the "Group"-Directory. However, it doesn´t write any Cachefiles into that Directory. I don´t know what´s wrong, everything works fine an the graphic is still displayed. I´am using the actual PEAR Cache Version (1.5.3) Any ideas what i can do or what´s the mistake ? I also use the Output Buffering Cache for whole Sites. This works fine - that means - it creates Cachefiles - only the Graphiccache Functions seems not to work. Thanks a lot - best reagrads Michael Frank