I have also had trouble progressing through Sweet Spot workouts as the intervals become longer and rests become shorter. I can maintain a longer Sweet Spot or Threshold effort in a race or on a climb but have difficulty completing harder and longer workouts. It is likely due to limited muscular endurance and inconsistent motivation. I don’t think it is an FTP issue as the Ramp Test and other tests and apps consistently give me a similar FTP.
Given the current WFH situation, I have built the plan to address my Sweet Spot shortcomings. I will be able to do TSS of 400+ and will intersperse Zwift group rides and shorter Zwift events during each week. There is a steady progression in my plan and if I fail a workout or have to turn down the intensity , I will be able to step back and pause the progression or take a recovery week as needed.
Abbey 6 x 5 min w/ 3 min recovery 62 TSS
Tunnabora 5 x 7 min w/ 3 min recovery 65 TSS
Ericsson 4 x 8 min w/ 3 minu recovery 64 TSS
Carson 6 x 5-7 min w/ 2 min recovery 66 TSS
Monitor 6-6 min w/ 2 minute recovery 65 TSS
Geiger 3 x 12 min w/ 3 min recovery 67 TSS
Antelope -5 4 x 10 min w/ 3 min recovery 70 TSS
Carillon 4x10 min w/ 1-5 min recovery 68 TSS
Tallac 3 x 15 min w/ 3 min recovery 74 TSS
Antelope 5x10 min w/ 5 min recovery 96 TSS
Ericsson 6x8 min w/3 min recovery 97 TSS
Tray Mountain 3x 20 min w/ 5 min recovery 103 TSS
Eclipse 3x 20 min w/ 7 min recovery 104 TSS
A Race - Wright Peak 3x 30 min w/ 5 min recovery 145 TSS