Where are you having the problem? Is it playing with a band? Timing? Or tone?
I Shot the Sheriff is not an easy song in its Bob Marley version (Eric Clapton's is a bit easier!) It's super-busy. Bill Laswell made an interesting observation about Familyman.... he's locked in mostly with Bob's vocals.
The key is to listen to a lot of reggae. It follows a lot of rules that blues based music follows so people think it's easy... but unless you know the music inside out, you're going to sound a bit fake... One thing to get used to is the one drop (not universal, admittedly). The drummer does not place emphasis on the "1", which might give you trouble... you just have to get used to that.