Why is time machine trying a full back up instead of the incremental changes?

I've looked for a good couple of hours and I can't find anything that helps, that I understand. I bought a 1 TB Seagate hard drive to use for the Time Machine. It backed up the whole system without issue, 600 odd GB leaving me the rest. Now, instead of saving the changes I make it constantly tries to do a whole system save at a further 600GB. Obviously there isn't the room and I'm sick to death of seeing the warning/getting grief off the wife for spending the money on the drive in the first place. Any assistance greatly appreciated, none techy if possible. Thanks.