if the dogs keen to chase and taking for granted the dog is fit?how many times have you gone out and caught nothin, that can become a bad habit,you and the dog have just picked up,but as that can be a bad habit ,gettin it right can be learnt ,dont slip the dog till you know its got a better than good chance of success,slippin it just for a run is no good, confidence is lost.you need to build it slowly, id do nothin with the dog for a while till it gets bored,then start properly.