10.5 Can't change partitions anymore.

I once created a partition and now need to make it bigger. But my every attempt to do so, or create a new partition, or make my main partition smaller has failed. It maps partitions and seems about to complete, and then the same error message every time. Not enough space on drive. There's over 20 gigs free so that can't be the issue. I certainly never had these problems before. I tried changing the partitions from a boot disc to the same result.

Is that 20 GB free on one of the partitions or is it 20 GB total free space for both partitions?

It's part of the 37gigs for the main partition. I made a second that's just a gig and half for important docs and the like and I just want to resize that up to about 4 gigs big. But I can't downsize the main OS drive, or make the second partition bigger, or create a new partition in the free 20 gigs of the main partition. Whatever is at issue didn't used to be. I have in the past made and deleted other partitions, changed sizes and all the like just how it's supposed to work.