Fermentation stopped after 3 days

I have an oatmeal stout in primary with an OG of around 1.7. I made a 2 liter yeast starter from a Wyeast smack pack and its been in the carboy for about 3 days. The krausen only ever got up to about an inch the first two days and now it is gone completely. Airlock activity around every 6 seconds though. I've never had a beer have this little krausen. Should I be worried? My starter seemed to grow a lot of yeast so I don't think the yeast was bad, but I'm getting concerned it might have given up. I did use potassium metabisulfate to treat all of my water before brewing which I have done several times before. I used a little more than necessary. Probably equivalent to around 3/4 a campden tablet for 5 gallons about an hour before boiling. From what I've read, this shouldn't be enough to kill my yeast and should have had time to do its thing. What happened to my beer? I'm confident my sanitation was better than average as well. Should I buy more yeast and add it?

Woah 1.7 OG, you throw the yeast into straight sugar! Anyways, assuming you meant 1.070...Have you checked your Gravity? Very possible the beer is close to, if not, at final gravity. When you pitch the appropriate amount of healthy yeast, like it appears you have done, beers will reach final gravity in 3-5 days. It is recommended to leave the beer on the yeast for a while longer as it can still "clean" things up. What yeast did you use? Sounds like a high flocculating strain. And what temperature is it at?

Haha. Yeah. 1.7 would be somethin else. Haven't checked the gravity yet, but probably will after another couple days. I used WYeast 1084 Irish Ale Yeast. Fermented at around 65 F. I am still noticing airlock activity about every 6 seconds or so, so I guess its still doing its thing. This is just the first brew I've had with so little krausen. Could be the slightly cooler temps I started with I'm guessing.