I wouldn't say Chester failed with Gorillas particularly. Of the four that reached adulthood at Chester, only Noelle, the female Eastern gorilla, died there at quite an early age. Although the Lowland gorillas 'Jason' and 'Gogal' never bred, few zoos understood the complexities of Gorilla relationships in those days and efforts were made to remedy this failing, both at Chester(with Belle Vue's male) and also when the pair were (seperately) sent permanently to Bristol zoo in their latter years to try and remedy this (unsuccessful as it was) Mukisi, the male Eastern gorilla lived at Chester a very long time in excellent health, before finally being transferred to Antwerp. So I'd say their record with this species wasn't bad for the era they were kept in.
Despite several calves being bred, I'd be more critical of the overall breeding record for the Black rhinos.