When you are doing short intervals (15 to 25 seconds), it is not often you will be able to get your heart rate up to Level 5.
Level 5 is the highest intensity possible, and more refers to the pace you should be targeting, which is a full out sprint. When done properly, you simply cannot sustain the speed and turnover for more than 20 seconds. With any intensity, heart rate lags behind. It is okay and actually common to not reach that max heart rate in that short period of time. One of the few times that you will actually reach that max heart rate would be at the end of a race or in an interval session that accelerates to a higher intensity at the tail end of the interval.
Focus in on your anaerobic speed, which kicks in after your aerobic systems cannot supply ample oxygen. The heart rate should not be the focus of your Level 5 speeds.
Typically, we tend to lean towards Level 5 intervals from 15 to 25 seconds and around 90 seconds to 120 seconds for rest. That way you can get your heart rate up sufficiently and also allow for full recovery in between repetitions.