Middles can get sloggy and boring. It's why I quit NaNo on the years I didn't win, and one of the main reasons the Prep was born. It's easy to lose steam when you don't have a strategy planned in advance. Is there anything about the journey to the climax that could be left out but implied and assumed by the reader? As H❀pe pointed out, adding complications, which can be resolved via literary devices, are a great way to get readers reinvested in your characters.