DLA Time Limits

Wondered if anyone knows what you write on application regarding amount of time you have had change in your circumstances.
I wrote on my application sent in august that my change was from February this year. I got my award but they only paid it from the date of my GP letter in august !
They said I had not notified them sooner.
I had previously applied last December and was turned down in January this year.
I thought you had to have had the change for 6 mths before you could apply.
I have asked for it to be looked at again as did not think this was fair.

Could anyone advise please.