Heart rate not measuring like it used to

Hello All,

I love using my Fitbit. When I started my fitness journey, I read that people that lose weight and keep it off tend to exercise more like 60 minutes a day. So at that time I bought a Fitbit and used the "active minutes" feature to measure how long my heart rate got up each day. My goal was 60 minutes, and I achieved that with 30-40 mins of FB and 30-20 minutes of walking or biking.

Now, it seems a lot harder to keep my heart rate up (according to my Fitbit.) I'm doing Fit R2 and a 50 minute level 4 workout will only get me like 20 "active minutes," meaning my Fitbit only measured 20 minutes that my heart rate was up.

I'm wondering if this change is because I need a new Fitbit, or maybe it's because I've been exercising so much over the past year that it's just a lot harder to get my heart rate up and keep it up.

Any thoughts?