In many cases you will need to be very aware of what you are actually downloading when it comes to ABR's (brushes).  Most brushes out there are completely useless for the purpose they were intended. If you want something specific it is probably quicker and easier to make it yourself.

OK....thats my lecture over..lol

Grass in real life is 3 dimensional, light passes through it and individual pieces of grass are fairly fragile to get some authenticity into your work you need to reflect some of these aspects.
If you duplicate your grass layer once, twice maybe three times it should (if you are using a quality brush) begin to take on a life of its own.

I have knocked up a couple of quick examples using my own GRASS ABR, just to give you some idea on how things can look.

Thanks to my friends over at CG ( the professional artists lifeline) for backgrounds...