Jun and searching4dan you have to take a look in this post, there is a draw whit the explanation that what you have to adjust by not gluing up
I think you have the skimmer inside pipe to large, it have to be no more than 5 cm down the out pipe (the central T elbow)
I had have the same problem and I cut the central pipe and put same conectors to enlarge it and the flow go up, maybe you have to try that.
hope it help
regards