tax credits potential payment withheld??

Hi

I recently cancelled my single tax credit claim on 19th april and submitted new joint claim right away the claim was received on 20th april anyway during processing of claim my partner is now unemployed due to ill health and Im going onto to mat pay so our income for year 13-14 has dramatically dropped, our joint claim has now been processed with lower income for the year but the money is not being backdated to the 20th april they have told me there is a potential payment sitting on our claim and if circumstances dont change we will receive it at end of year? I have had no money coming in and now and not receiving back payment as planned.any advice?

Thanks