Has anyone ran across the Air Valve Assembly getting stuck every so often on these mixers?

We have this problem with two different customers, installed a new one, and it worked fine for a while, then a no start, we go there and find that the AVA is stuck again, not letting any air pass by. We can clean the throat, and use some spray lubricant, and it'll work fine for a couple or so months, then do it again.

We did talk them into changing their LPG supplier, and it did make it last a lot longer, but today, we get the dreaded call from our customer.

Any idea how to stop the AVA from sticking?

Thank you.

CA55-577-H25 LPG Carburetor Mixer