When I use my FR55 with an HRM200 chest strap, it doesn't read my heart rate

Hi everybody!

I've been using my Garmin Forerunner 55 for over a year, but I decided to start using a chest strap to have a more accurate monitoring of my heart rate. However, it doesn't seem to work and, since I'm pretty useless with technologies, I figured I was doing something wrong. 

- My watch detects the strap and it says "connected" when I check.

- I've made sure both "Garmin" logos are facing the same way.

- I've made sure the sensors are a bit wet.

And yet, if I run using the strap, it doesn't read my heart rate. I finished my run and no data was available. I tried again next time and same thing. If the strap was on, it gave me no data. I had to switch off the strap so I could get the data from my watch directly.

Does anybody know what I could be doing wrong? 

Thanks!