I've had similar problems using qparted. It doesn't seem to have the full functionality of parted.
My suggestion, try the command line version: parted. Or perhaps cfdisk.
Note: if you are successful at making size changes, before you do anything else, reboot.
I once made the mistake of not rebooting after partition changes, and ended up with overlapping partitions. My only option was to delete the partitions I'd created/resized, and do it all over again.