Most rabbits know what to do, or at least figure it out pretty quickly.
Since your buck (and you) are new to this, I feel like I must ask - do you know what a split penis would look like, and have you checked to make sure he doesn't have one? That is what I was hinting at when I said "mechanical problems." Splits are disgustingly common in some breeds (particularly the smaller, "pet" breeds); I would never buy an unproven buck without checking to make sure he doesn't have one. To check, flip him over, and do the thing most people do to check the gender on baby rabbits. If his penis stands up straight, he probably doesn't have a split, but if it curls, you need to look closer. Some bucks just have a little split right at the end, but some are split all the way down. Either way, the curl can interfere with his ability to "get the job done," which sounds like what might be happening here.
Since your buck (and you) are new to this, I feel like I must ask - do you know what a split penis would look like, and have you checked to make sure he doesn't have one? That is what I was hinting at when I said "mechanical problems." Splits are disgustingly common in some breeds (particularly the smaller, "pet" breeds); I would never buy an unproven buck without checking to make sure he doesn't have one. To check, flip him over, and do the thing most people do to check the gender on baby rabbits. If his penis stands up straight, he probably doesn't have a split, but if it curls, you need to look closer. Some bucks just have a little split right at the end, but some are split all the way down. Either way, the curl can interfere with his ability to "get the job done," which sounds like what might be happening here.