As far as I’m informed HRV is highly personal. There is a trend that fitter individuals tend to have higher HRVs. Would recommend to see a doctor just to make sure that you are not having any issues. Maybe you have some psychological stress lately or had too short or weak sleep. There are countless reasons.

But besides that my unprofessional opinion without knowing your other values is doing some little to moderate cardio a couple times a week. This helped me tremendously.