Some species can take an exceedingly long time, I had a female P. alleni take several months to berry once. And it can take longer if you aren't separating the male from the female after mating (he'll just keep replacing the sperm plug over and over again, which can prevent berrying sometimes). Basically, I'd just recommend letting them mate, then isolating her in a tank with a lot of hides and waiting a month. If nothing has happened, try again and if nothing happens still, try with different individuals or different species.
