Euro-reef RS 135 setup?

[Neil]

i have had an euroreef rs 135 on my tank for a few weeks now (with sedra pump). I find it skims really nicely- sometimes. im constantly adjusting the riser pipe to get the skimming bubbles at the right height (sure the water level can change 1-2 inches in my sump due to evap over a few days). Today for example, i go downstairs to see the bubbles just in the base of the neck, so i adjust the riser to get them to about an inch lower than where it spiils into the cup, so its skimming for a while, and then i come downstairs and its not even bubbling anymore- its just churning water with about 1/4 ich of bubbles ontop of it about half way up the neck. the venturi hose is clear- i hear it sucking air, valve is wide open, i pulled the inlet off the pump the other night to see if anything was obstructing it- and no. It seems hit and miss, sometimes not even making foam. i use prime conditioner btw. I have read many things about these skimmers (and none bad)- mainly that they are advertised as plug and play- well, im done playing- i just want to plug and skim. Any advice from exp'd ER uses?