Shooting in the gi is pretty hard if you're just trying to use generic wrestling setups. As noted, it's pretty easy to stiff arm people away and prevent the shot, and achieving any kind of separation is just really hard with the cloth. As such, you really need to use the gi to setup the shot and don't expect to get the same kind of penetration you would in wrestling. Collar drags to single legs work really well, as do ankle picks since you don't have to get in on the hips as much. Firemans work better in the gi in my opinion, because you can create so much pull to enter. A lot of times the grip you'll want is one collar grip and to lock down the sleeve on the other side so that when you go to shoot uke won't be able to block your entry. Get the sleeve grip locked down as priority #1, and then get your collar grip and attack.