VO2 Workout and Heart Rate Questions

I needed a couple of recovery days after the Saturday/Sunday combo. That put me a bit behind schedule for the week, but I wasn't able to give a good effort for a VO2 workout before this morning. It's okay, even desirable sometimes, to go into a long run with tired legs. However, it never makes sense to do a VO2 workout with less than fresh legs.

I did a 5+ mile warm up followed by 5 x 600m repeats with a 200m recovery jog. My goal for each of the 600m was sub 2:10.

1. 2:10
2. 2:08
3. 2:08
4. 2:08
5. 2:05

I probably need to figure out how to run those nearer to 2:00. They felt hard. I was uncomfortable. I was struggling to maintain the sub-6:00 pace on the last 100m or so of each repeat. My feet felt heavy and I felt like I was landing hard with each step. But my heart rate was lower than expected for those. My max heart rate during the entire workout (177) was equal to my average heart rate (177) during Saturday's 10 mile race. Hmmm.

My last VO2 workout was similar. I ran at cookie tossing pace last time, but my heart rate never got above 179. My heart rate has regularly been getting in the mid-170s during tempo and MP runs, which are 0:45 to 1:15 per mile slower than my VO2 intervals. For reference, my max recorded heart rate was last summer during a hard workout at 189 and my resting HR is 39.

Is my HR monitor defective? Am I not putting enough effort into the VO2 workouts? (Isn't dry heaving a reasonable indicator that I gave good effort?) Is my tempo and marathon pace too difficult for my current level of fitness? Do I need to do more speed drills so that I can go faster and push the heart rate up during VO2 intervals? Or do I just need to forget HR and go by feel and pace? If you can answer these questions, please do!