Well, this is little bit too broad because there could be a numerous reasons for this, but I'll try to narrow down.
You are banned.
From this article here you can see many reasons for account being removed / banned etc.
If your stumbles are not showing up in your SU profile, your account might be banned. Here are some possible reasons for banning in SU.
Too many different SU usernames voting from the same IP address.
Reciprocal voting activity, based on tracked patterns or published confirmation (i.e., a blog post or social media campaign suggesting
potential reciprocal voting activities)
Too many users voting on the same story and coming from the same referring URL – e.g., from a forum listing.
Misuse of the ‘send’ button. The SU browser tool bar has a Send button that lets you message your SU friends on some content you’d
like them to look at. If you you’re only sending them your stories,
votes for your site could be discounted.
Complaints. This is a pretty broad area, and there can be any sort of complaint from other users which might cause you problems on SU.
StumbleUpon is wonderful for traffic. Try to contact them directly to see what's up with your account.
But, from my experiences, I know that in first week or two I get many views. Eventually I'm getting lower and lower and lower views. You need to put fresh and new articles over and over and over again, and that's it. This is not unusuall. I thought that you were banned. But, this is (from my experience) normal behaviour. It's stumble upon dude. I don't use it so often as I used to do. I don't know what else to say about this matter. It site for generating traffic.