Does anyone know how to encourage my bristlenoses to lay eggs???

Hi, Ive already posted one in the general disscussions, but i have tried all that stuff, and it just doesnt work. has anyone got any theories that bristlenose plecs just cant resist spawning?? i really need any help i can get with these guys because they are my first and theyre certainly not making it easy for me!!! please help!!!???!!!

hi mate Ive just got my bristle noses to breed and it took 3 years i introduced a lot of caves slate ones and constantly fed them greens like courgette and cucumber and veggie wafters and also changed 20 per cent water to 4 degrees colder than the tank and also have the water temperature at 72 degrees do water changes weekly and sit back mine are laying that many eggs at the moment that i could do with another male.