The group, which teetered on the brink of insolvency in 2009, said sales at stores open at least a year rose 18 per cent from August 2 to August 29, but had slowed to a six per cent increase from August 30 to 26 September.
"There appears to be increasing caution in the market over discretionary expenditure which means that it is very important to attract customers to our stores with a good range of products, services and prices," it said in a statement.
JJB said customers had responded well to promotions, but sales growth fell back when they ended.
The group made a loss before tax and one-off items of £21.9m in the six months to 1 August compared with a loss of £46m in the same period last year.
Like-for-like sales for the year to date were up 13 percent, with cumulative gross profit margins in line with the half year. JJB avoided administration last year by selling its fitness clubs, pushing through a debt restructuring deal with creditors, raising £100m of new capital and renegotiating banking facilities.