El Capitan Disk Utility won't resize internal SSD

After I originally upgraded to El Capitan, I used Disk Utility to resize my partition from 500GB to 329.47GB. This worked fine and did not delete my Recovery partition at the time.

I now need to further reduce the size of my OS X partition to fit another operating system, but Disk Utility will not reduce the partition size even thought it says it successfully has done so. Note that there are no other partitions on the disk; just empty space (and sadly no Recovery partition, which Disk Utility deleted for me the first time I tried to further reduce the partition's size).

I need to mention that I have rEFInd installed (mana for the trolls), but that should have nothing to do with partitioning a drive, and as can be seen, I am able to boot into OS X using rEFInd.

Here are a series of screenshots showing the stages:

Open Disk Utility

Select the SSD and set the size to 300GB.

After clicking 'Apply':

The size is still the original 329.47GB, even after rebooting. Finder also reports the 329.47GB size, so clearly Disk Utility did not do anything (other than delete my Recovery partition).

Am I doing something wrong?

The product code reported by the Web Page is incorrect as I have El Capitan running, not Yosemite.