Mob spawning is too slow

Hello, I play Minecraft Windows 10 Edition with a friend. On the realms, we are going to the nether and the spawn rate of the mobs (especially of the skeleton wither) is extremely low. It is therefore almost impossible for us to farm the withers heads... We have a free trial realms and we were planning to pay a 2 player realms to continue to obtain the successes of the game. If the spawn rate is so low, as much hosted the game ourselves it will be much more efficient ... And also another small bug is when I get a success, I do not notify, or, my friend, can not win ... Hope to help you by pointing out these various bugs. Sincerely, Rémi, Minecraft player ^^