Struggling with zone 3

I am following the triathlon maintenance plan using heart rate for my zones and for the last couple of weeks I have been struggling to get into zone 3 when doing cycling intervals. I can feel the burn in my thighs and cannot increase the pace and maintain it for the required time. I end up riding zone 3 intervals in zone X. It feels like my lactate threshold heart rate has decreased. What is going on and what can I do? Thanks