One thing that makes meleeing hunters hard is the lag and hitbox calculation that changes from server to server. For example, in a lag free environment, its easy to do so as long as you have the timing and distance of your melee down. In a lag inclusive environment, such as 50ms, or more commonly 100ms, this becomes significantly harder. You can typically judge how much lead time you need by shooting at a hunter who is pouncing at you with a shotgun, or autoshotty. Typically middle of the cross hair shot should put the hunter down but it either completely misses or nicks the hunter. Or you can observe the time it takes for your hunter to land the pounce after you've accurately pounced on someone. What this means is you should notice a delay time for your hunter to land on the survivor after directly hitting them. Usually this is seen with the hunter hands moved up in a pronounced fashion after you've squarely landed on a survivor. Once you've gauged the delay time, you can then utilize that which is missing in your melee calculation, or timing. I assume you are only having an issue with the hunter.