Do you use time machine?
Maybe you can try this:
use the command "sudo tmutil disablelocal" from terminal
TM will do that on any laptop if it is not tethered to a Time Machine drive. It then deletes the files (local snapshots) later - see some info here:
Try restarting the computer, then select the disk's Desktop icon. Press COMMAND-I to open the Get Info window and check how much free space is shown. If it still hasn't changed at all then try the following:
Repair the Hard Drive and Permissions - Lion/Mountain Lion
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported then click on the Repair Permissions button. When the process is completed, then quit DU and return to the main menu. Select Restart from the Apple menu.
Although the suggestion to delete old Time Machine snapshots, if any, that will only free up space but will not fix the problem you have described. If you are deleting files but the free space on the drive fails to update, that means there is a directory problem on the drive that needs to be fixed.