that's likely all you will receive if it said it was ftb supplement
there are a number of reasons you may not get as much - quite often this one is where you have over estimated your family income (say you put combined income at 100k for example) but underestimated severely the lower income earner and ended up with an overpayment of ftbb (so something along the lines of estimated 5k for one, 95k for the other, but reality being that your actual incomes were 40k/50k so all the ftbb you were paid was overpaid even though your total estimate was less than the 100k)

if you are concerned about being underpaid you'll either need to give fao a call or wait for the letter to come out explaining why you got that amount of moolah