Windows Server AppFabric 1.0 (or Microsoft AppFabric for Windows Server as 1.1 is now called) provides feature to enable local caching for enhanced performance. There can be a situation when AppFabric client could start deleting local cache much before expected (that is before configured Timeout, or Notification). One of the reasons of this could be if virtual bytes (total of virtual address space memory actually used by process) of client process touches 85% of RAM on machine.
To check if you are running into this situation, enable ‘Virtual Bytes Peak’ performance counter on AppFabric client process and graph it against total Physical Memory on client machine. If web application is your AppFabric client, the client process would be worker process (w3wp.exe) corresponding to its application pool.