Lactic threshold (LT) not updating

My Lactic threshold (LT) hasn’t been updated in Month. I run all my workouts with a HR monitor and have been doing interval training and long runs.
(yes it is set to ON – auto detect)
This morning I tired the watch’s (FR 935) test this morning and after going thought the test it said 'Threshold not detected'.
I know my TR has changed cause I am now running faster in the same avg HR so not sure why it is not updating.