Plecos have never really been bred in a normal aquarium tank. The need like 200+ gallons, current, lots of plecos to choose from, and lots of caves and junk.
Bristlenose plecs ( ancistrus species ) are very easy to breed.
To get a pair buy at least 6 induvidual fish,prefferably from different sources to avoid getting fish which are siblings.Feed them well to get some fast growth going and soon the strongest male will grow his bristles.Then just wait until he pairs off with a female (presuming you have one ).Once you have a pair remove the other four (either to other tanks or return them to the lfs).
To condition the pair to spawn feed lots of bloodworms and courgette/cucumber and perform daily large water changes,it shouldnt take long before they get down to business.