45 minutes borderline threshold

A repeat of the session from last Thursday. A 2km warm-up, then 45 minutes threshold, then 1.9km to finish with. Struggled to get into my threshold zone and I was borderline for the first half of the run. Very frustrating as every time I looked at the watch the heart rate was always a beat or two lower than where it needed to be. I finished up running 13kms in 45:30 which was exactly the same time as last week.