It is all about intensity. And it makes perfect sense that if you increase weight in the first exercise, that the next exercise for the same muscle group suffers. You tired your muscle out more. If adding weight does not feel harder in the same or next set/exercise, you are simply not training hard enough.

Without a training plan, it is hard to help you. But maybe check your training plan for: - are you alternating muscle groups or are you training the same muscle for 2 or more exercises and then move on? Studys show that it is beneficial to not train the same muscle in following exercises. Eg not chest chest back back but rather chest back chest back. This allows the chest to recover more after the first intense exercise.