Ratio Strain; where the amount you are asking of the learner exceeds the reinforcement available and they have long pauses in responding after the reinforcer is given (post-reinforcement pause) or they stop training altogether.
This can often look like boredom, lack of interest, walking off, easily distracted etc. In fact it is that the shaping is in steps that are too big or the amount of energy required exceeds the value of the reinforcer on offer.
This can be remedied by decreasing your shaping criterion and/or increasing your rate of reinforcement.
Fear Free Training with your horse 