My site got google ranking and google send to me xxx traffic.
but yahoo only x traffic.
it's been a long time.

i think it might be yahoo too weak not enough traffic send to us.
not our ranking too weak in yahoo search engine.

how do you think

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.

Do not compare yahoo with google both are different search engines to get traffic from yahoo follow the yahoo search algorithm.

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.