Are you experiencing less performance for the same activity or segment of that activity? Remember that as your cardio vascular system gets better, you won't need as much HR to get you up a hill as you did when you started out measuring this stuff.
If you aren't actually going out and doing a lactate threshold test for 20 minutes or whatever higher, then I'd be leery of what the LTHR calculated from data points of activities where we aren't trying to go all out.