Firstly are you going to Navigate To/ Address/ Postcode then entering the full postcode as this is the correct way to use the device, enetering a postcode into where the device expects an address will often cause the device to only use the first three or four characters.
If the above doesn't fix the issue try this:
There should be a folder on the device named "zip" in this folder should be the postcode data files. If its missing you should be able to add the necessary files using TomTom Home, or if you want to update it using Windows Explorer download the postcode files from
http://www.tomtom.com/updates/zipcod...ipcodes715.zip unzip the download and put the zip folder in the root of the devices memory, your postcodes should now work - Mike
tomtom one v4 
Linear Mode
