Do you miss a lot of last hits? Your GPM is ridiculously low. Take this match for example:
http://www.dotabuff.com/matches/1067212144

This player on the other hand, is rated 5.6k and is one of the best TB players in SEA:
http://www.dotabuff.com/matches/1094362875

Note the game duration and the difference in how many last hits were obtained.

Not too sure. I don't usually look at gpm as a support, just xpm and the progression of my cores. If I can secure about 2k farm on my carry in about 4-6 minutes, job's done.

Not sure if it's good practice or not, but I only rotate when it's time to check for runes. I learned the hard way that rotating too much without a kill results in a support falling too far behind. These days I tend to play a little more selfish and only roam if I can secure a kill/assist. If I see a core dying but I prevent the death, I don't even bother TP-ing.