For years I have cleaned the mounting areas FOR and OF ALL the zincs, shaft zincs, plate zincs, all zincs and so on. I then always check with my ohmmeter for continuity of less than 1 ohm, hopefully in the range of less than 0.5 ohm. Several times I have to dismount a zinc and reclean the zinc AND/OR the area it is mounted to.
I have seen zinc protection failure unless that is done and especially the ohm check.
Just fastening a zinc in place is NO guarantee it will actually work if the mounting surfaces are not adequately clean.
For pencil zincs I use a wrench to hold the cap and a pair of pliers to SNUG the anode into the cap. I no longer lose the zincs.
I then apply some sealant to the cap threads and check that. I realize that the water the anode is immersed may create a pathway to fool one but I have also checked when the raw water system was drained.
Every 6 months or so I then check for anode activity with my silver/silver cell and meter. I have caught a couple problems before they could cause damage.