So you've been working hard, you feel like you've been getting faster, but when you retested your FTP actually dropped...WHAT?!
There are a few reasons this can happen:
It is likely that you are carrying more fatigue in your legs than when you completed your initial FTP test. In these cases, your FTP actually has improved, but your fatigue is preventing you from demonstrating your gains. The best thing to do is give yourself an extra recovery day or two before trying that FTP test again
Poor Mental Preparation
Oftentimes, especially after an easy recovery week, we can feel a little "flat" both physically and mentally. Essentially, we've gotten used to the soft, comfortable rest week exercises and we've forgotten what it's like to be in the pain cave. If you think this could be the case, it might be better for you to switch your Tuesday and Thursday rides. In other words, you would test on Thursday after reminding your body what "real" training feels like.
Going into an FTP test where you feel like you've made big gains, it's easy to overestimate yourself and burn yourself out too early. This will be easy to spot because when you look at your graph, you'll see that you started out like Chris Froome, but then the power started to drop steadily until you're putting out a fraction of your initial power. On the other end of the spectrum, if your power shoots up in the last few minutes, then you had too much left in the tank and your test results would have been higher if you were to pace more evenly throughout the interval. To solve this, simply look at the graph, identify your mistake, and attempt to flatten out your effort for the next time.
Had an Off Day
Work stress, family stress, poor sleep or any number of real-life struggles can wear you out and make it harder to perform your best. In these cases, just give it a few days and give it a try on a better day for you.
Performing an FTP assessment (or any highly demanding workout, for that matter) can be especially difficult on the heels of a recovery day. And while it’s a good idea to be rested and refreshed prior to assessing your fitness, many riders benefit from a “priming workout” the day prior. Adequately recovered riders often use these on the day preceding a race in the Specialty phase for the very same reason: to prepare the mind and body for the abuse to follow, and to do it without hampering freshness.
Takeaway: There are a variety of reasons why you may have had a bad FTP test, so don't freak out. Reflect on your performance and see what aspects were lacking and work to improve next time.