Unlike counting steps, in order to accrue Strain, you must have an elevated heart rate for a sustained period of time. Within that elevated heart rate, there is a minimum heart rate-reserve threshold in order for you to receive Strain. If the minimum heart rate reserve threshold hasn't been met, you will not begin to accrue Strain for the day. This is very common early in the day and if you are taking a Rest Day or Active Recovery Day.
To start to accumulate Strain, you need to reach 30% of your Heart Rate Reserve. Your Heart Rate Reserve is the range from your resting HR to your max HR. For example, if your max is 200 and resting is 50, you have a 150 bpm reserve, so 30% of your HRR would be 95 bpm, or 50 + 0.3 * (200-50).
In this example, your heart rate would have to exceed 95 bpm for a sustained period of time in order for you to begin accruing Strain.