Likely your body is just not ready for a VO2 efforts. I ran into the same problem coming off traditional base and going into general build. I had a FTP gain of 8% at the beginning of general build, but struggled heavily. I ended up reducing my FTP manually by 4%. I was able to finish the remainder of the weeks in GB. The 4% still allowed for challenging workouts, but it didn’t bury me or cause me to reduce the intensity or duration.