@ jase
The last week of SS base is a recovery week, so you should go into the build phase, which starts with an FTP test, quite well rested. That’s the theory.
I assume that you’re doing the low volume plan?
Are you doing anything else like gym work, football, normal riding that is causing excess fatigue?
I found the 90 minutes sessions hard in the second block, and there is a subtle move towards higher intensity VO2max work in that plan too.
I’ve done their SS base a few times now and do feel that that by concentrating on the specific aspects of base fitness I can then move on to the more VO2max work and really push myself.