sounds like you need help with setups more than you need help with an armbar. There's lots of ways to set up an armbar from the mount. No gi is a little limited, but you can still go in and work an "ezekial" type choke, I really don't know what it's called, but it's pretty much cross facing into a forearm choke. alot of times people will give you an arm off that. Also, what I've found to be really REALLY effective was a move one of my training partners showed me because we're around the same size.
Take an arm and shove it to the side hard, like you're going for an americana. his first instinct is going to be to roll onto the side of the pinned arm, buck, or try and muscle the arm back to the center. You can get a submission from either of these situations.
If he rolls to the side, switch your hips, go to s-mount, and take the arm opposite of the one you pinned for the americana. You don't really have to worry about him bucking here because he's pinned pretty well and he's on his side.
If he bucks - ignore and americana him.
If he tries to muscle it back, let him, as odd as this sounds, I always watch their face to see if they're going to muscle. For some reason, the guys who try to yank it back always get this constipated look on their face right before they do it. So I'll pin hard, see that they're going to jerk it back up, and I'll pin them until they start, once they pull I give 0 resistance, the arm will usually come right up, and usually they kind of chickenwing their elbow out. So even if you can't switch your hips in time to get the armbar, you can very easily scoot up on him from normal mount position and get your knee seperating his elbow from his body, from there it's just a matter of time.
