As you get fitter your resting heart rate gets lower and thus it becomes much more difficult to reach the same high heart rates you were able to reach before with less difficulty. You do have to push yourself harder to get to 80% of max and beyond. You will definitely feel it, trust me.

But that doesn't mean you're not burning as many calories. Your body is supposed to be burning calories more efficiently the more fit you are. But your HR monitor is only recording the lower heart rates you have which translate into fewer calories burned overall. Push yourself harder or change your workout to reach those high heart rates.

I myself only burn less than 400 calories (450 if I really push it) per spinning session (no matter how difficult!) but that's because my resting heart rate is in the mid 40s. I am fit, but I also naturally have low resting heart rate.