Even my track intervals have been pretty weak over the last few weeks, both on HR-measurement effort and time, which suggests to me I'm not driving myself hard enough. How to relight that fire?
I've had this happen before, and it usually means I need some extra rest between hard efforts.
In other words, your legs may be getting a little bit stale from doing more or less the same thing week after week. Perhaps when this happens, you could try inserting a little rest or a change of routine, like fewer intervals and more long, easy stuff.