It's hard to say without watching a clear video of your rotation, but SMN is a rather complex class and can be a bit overwhelming at first. In general it sounds like you're doing the right things, such as waiting for your aetherflow to refresh for your first Bahamut stage for the festers, keeping on top of your dots...

I think the easiest thing to miss as SMN is weaving in skills. For example, I see a lot of SMN's casting Ruin 3 - Fester - Ruin 3 - Fester, when in fact there is time to get the extra dps of doing Ruin 3 - RUIN 2 - Fester - Ruin 3 - RUIN 2 - Fester, etc. Also, in terms of AOE, be wary of when you use bane. As it's potency decreases with the more enemies, it can often be a lot more useful to painflare instead.

But, like I said, it sounds like you're on the right track so if possible try to record a video and I'll give my best advice!