How to find out the insertion loss of a stub tuner?

I started to use the stub tuner (3 step) to find out whether my PA match is optimal one or not, and found that the o/p power is 4dB lower than that of without tuner present at PA o/p at different stub lengths. Just wanted to know whether the insertion loss of tuner is this high? I doubt this is the case, if so how to find out the insertion loss of the tuner? I am doubting this would have got damaged while shipping as I find the silver coating on the body of tuner is discontinuous at few places.

If the source and load impedances are not the same there will be a Missmatch Loss present in the system.

ML[dB] = -10*LOG(1-Γ2)

Even using an ideal tuner with no insertion loss, a loss will be present due to different source and load impedances, loss which tuner tries to minimize.