For me it would depend on why you had to skip service. If we had to skip because of aggressive dogs, or the customer leaving the gate locked even though we provide ample "On The Way" notices before arrival, I would still charge them the full amount because you spent the time driving to their property and the onus was on them to give you safe access. If it's things out of your control like weather or a flat tire for example, I'm more willing to provide a credit or discounted service because it wasn't the client's fault you couldn't service their yard.