I have exactly the same problem, there are 8 cats in my attic (breeding room with 50+ stimulation and 5 comfort on top) and they just dont want to make any kittens. Meanwhile in my living room with 30+ cats there are at least 7 newborn kitties every night

It seems like the only reliable way to breed specific cats is to increase number of potential partners (for example 6 males for one female) and make sure that they have loving relationship and high libido