It is possible to get pregnant on prostap, just like with on any hormone drugs, theres almost never going to be 0% chance, but it will depend on whether or not the prostap has stopped you ovulating (which it should once its shut your system down as that is how the drug works), hence the longer you have been on prostap the lower the chance of you still ovulating, if you've been on it more than 2-3 months then its unlikely youre still ovulating, the risk of pregnancy is much higher in the first month or so whilst the drug is getting in your system. Almost all of times I've come across women getting pregnant on it have been in the first 4-8 weeks.
On the other side, it would be really really unreliable to try to work out if you have a fertility problem based on not getting pregnant whilst on prostap, not much different statistically to trying to get pregnant whilst on hormone contraceptives! I can't imagine that there is any doctor that would be willing to rely on that as the drug should stop you ovulating - plus there is only really a 12 hour window in any month where you can actually conceive, and tracking that whilst also on prostap would be tricky
Try not to worry but if you are concerned do a couple of pregnancy tests a week apart so at least you would know early or speak to your gp x