They could be fat or have eggs and are egg bound. Tiger barbs (and other barbs and danios) need a temperature fluctuation to stimulate them into breeding. If they are kept in warm water all the time then the females often become egg bound and never breed. They should be kept in cooler water around 20C and when you want to breed them you can increase the temperature to 24-26C. You can also separate males and females for a week and then put them back together. This will often stimulate them and encourage breeding activity.