Your Yahoo SERP has something to do with your traffics from Yahoo. Of course, if you have high SERP, you have a high exposure and a high chance to see thru Yahoo search.
Didn't get if your rankings are good or not on Yahoo.
Yahoo and Google are different, so for the same keyword they will bring different traffic. If your rankings are high in both of them, probably you should look for keywords that bring traffic in Yahoo too.
Both and other SE use different algorithm, so don't expect same result for all the SE.
Find out the best keywords to focus on to get good ranking in the different SE. What is a popular keyword is SE1 may not be popular in SE2
You will typically get at least 5 times as much traffic from Google as Yahoo if you have good rankings on both search engine.
It should be very easy to determine if it is your rankings causing the lack of traffic. Just compare your rankings for different keywords on both Google and Yahoo.