Aqua medic skimmer air lock?

[Boxfish]

i have an aqua medic turboflotor 500 in my RSM rear chamber and recently after I turn the pumps on after feeding the skimmer does not always start up properly. Pump runs but no bubbles are being produced but I can see water in the breather tube surging up but it can get high enough to exit. I can jump start it if I blow into the tube. I have cleaned the impeller and housing and shortened to tube but it still does not reliably start up. I have mess around with the height but that did not seem to help, I can not adjust finely because I'm not using the orginal mount. Unit operatated fine for 4 months prior. Any ideas?

 

[twilliard]

Can you shoot a picture of the impeller and magnet?
I have had these problems in the past with skimmer pumps.
There is a reason they do this depending on how it's made.