How to clean carpet with different pile heights?

It's my grandma's carpet. I THINK its berber but not sure. She has a Oreck bag vacuum that doesn't seem to work so great, so my mom and I went over with my mom's Rainbow vac which usually works wonders. The vacuum attachment didn't work so well. I noticed that since the carpet has different heights, it couldn't get a good suction against the carpet. So we ended up using the hard floor sweeper attachment which took FOREVER because it is only 6 or 8 inches. I'd like to get something my grandma could use that would work better. Any suggestions?