You know how you don't usually think about your legs when you are walking, right? Well you also don't usually think about your voice box vibrating when you are speaking either, but now you should to an extent. If you notice that it stops vibrating when it should during an R sound, then you know the solution to the problem next time is to make sure your voice box is vibrating during those R sounds. Don't try to force it, it needs to sound natural. I like to just hum to myself sometimes to remember what the vocal cords vibrating should feel like. Then you combine that humming with moving your mouth the right way to produce the R sound, and then you are on your way. I realize it might sound a bit weird or complex, but I promise there is science behind this stuff.