autofs problem on RHEL 6

following problem occures on our RHEL 6 machines with autofs
after starting autofs all mounts for
/net//export/vobstore/01 .. 05 are applied.
after 5 minutes about, this is our expiration time on which autofs should unmount all unused mounts, it is not possible anymore to mount directories
/net//export/vobstore/01 .. 04

command output 'ls /net//export/vobstore/01'
ls: cannot open directory /net//export/vobstore/02: No such file or directory

cd /net//export/vobstore/01
ls
ls: cannot open directory .: No such file or directory

the mounted directory /net//export/vobstore/05 is still accessible because it is already in use
but it is not possible to go to one of the other (01 .. 04) directories

directories which has to be mount are stored on an NetApp system