Unable to Resize Partitions
If I understand your question correctly you are trying to resize the volume group or actually one of the volumes within a group. From what I could gather your disk was converted to a CoreStorage Volume. Could you please verify that by issuing the following command in a terminal and check if you get a similar output to the one in the picture:

diskutil corestorage list




The currently available Disk Utility doesn't support resizing of logical Volumes.  However using the command line, there is a undocumented function that allows you to resize Volumes. 

If you consult the help command of diskutil you will receive the following output:

chris$ diskutil corestorage

Usage:  diskutil [quiet] coreStorage|CS &lt;verb&gt; &lt;options&gt;,
    where &lt;verb&gt; is as follows:

 list                     (Show status of CoreStorage volumes)
 info[rmation]            (Get CoreStorage information by UUID or disk)
 convert                  (Convert a volume into a CoreStorage volume)
 revert                   (Revert a CoreStorage volume to its native type)
 create                   (Create a new CoreStorage logical volume group)
 delete                   (Delete a CoreStorage logical volume group)
 createVolume             (Create a new CoreStorage logical volume)
 deleteVolume             (Delete a volume from a logical volume group)
 encryptVolume            (Encrypt a CoreStorage logical volume)
 decryptVolume            (Decrypt a CoreStorage logical volume)
 unlockVolume             (Attach/mount a locked CoreStorage logical volume)
 changeVolumePassphrase   (Change a CoreStorage logical volume's passphrase)


As you can see, it doesn't offer any option of resizing a Volume, hence my guess why Disk Utility doesn't allow you to resize.

However the following undocumented functions exists:

    chris$ diskutil corestorage resizeVolume

Usage:  diskutil coreStorage resizeVolume
        lvUUID|MountPoint|DiskIdentifier|DeviceNode size
Resize a logical volume, which is one of one or more disks that consume storage
out of a logical volume group. The logical volume group will have more or less
available space after this operation, if it was a shrink or grow, respectively.

Example: diskutil coreStorage resizeVolume
         11111111-2222-3333-4444-555555555555 10g


The resizeVolume function isn't listed in the first listing of available commands above but it still exists and it even provides you with an example on how to use it.

With this command and the correct UUID of the Volume it should be possible to resize one of the Volumes within your logical group.

I did some more digging around the command line and came across the following commands:

Physical Volume (Disk) Commands


resizeDisk (undocumented) – Resize a physical volume 
removeDisk (undocumented) – Remove a physical volume from a logical volume group
addDisk (undocumented) - Add a new physical volume to a logical
volume group


Logical Volume Commands


deleteVolume (undocumented) – Delete a logical volume and all of its contents
resizeVolume (undocumented) – Grow or shrink a logical volume
resizeStack (undocumented) – Grow or shrink a logical volume as well as the volume group and physical volume.