You decide that you are going to condition your dog to salivate to the sound of a metronome. you sound the metronome and then several minutes later you give the dog a biscuit. you do this several times but no conditioning seems to occur. this is probably because

It was too long a time between the sound and the biscuit.

Explanation:

You are trying to condition your dog to salivate to the sound of a metronome by giving some minutes later a biscuit to the dog. If the time spent between the sound of the metronome and the biscuit is too long this can prevent the conditioning from occurring even if this was done several times. Conditioning is a process that consists of learning a behavioral response to a previously neutral stimulus.