It would help to know more about your split routine, how long you have been training, if you have been focusing on progressive overload, meaning, are you lifting more weight consistently or doing more sets, how long have you been doing this particular routine, etc. The body adapts eventually, which is good, but when it does, in order to keep making progress you need to offer it a new challenge. This doesn’t always have to be a complete overhaul of what you are doing. You body will tend to adapt to rep ranges quicker. The beginner may have to switch less often. If you are a bodybuilder or you have been strength training for a while, you are considered more “trained” which means you may need to make changes to your program as often as 4 weeks.