All of the above and Bashful +6 is tough. That was my first failed workout, almost certainly due to cumulative fatigue. I’ve had a couple of other failed workouts since then and attributed each to one of the items @mwglow15 listed. BTW, the occasional failure means you’re pushing yourself and probably have your FTP set correctly.
When was the last time you had a recovery week? Any time completely off of the bike? Have you been doing the same plan(s) over and over again, or do you have variety?
As for doing VO2Max work, several of the plans have good progressions in them including SSB MV and Sustained Power Build.