In the case of Twycross zoo there are absolutely valid reasons why the current tally is non-paired Amur leopards and Asian lions. What some seem to forget that those species are not exactly bred as a production line - that does not happen in wild either -. Besides, in a zoo environment we can make more informed decisions on which pairings are genetically suitable and relevant to the managed population.
For both aforementioned species it is not easy to find a suitable genetically distinct suitors: in the Amur leopard females are in shorter supply anyway + the number of possible founders is limited and in the case of the Asian lions it is the limited number of founders and the factual need for introduction of new Asian lion founders into the European population.
So, get real out there (not everything you wish for can be realistically achieved FOR NOW)!